Re: [PATCH 16/27] drm/i915/gem: Add an intermediate proto_context struct

[Jason Ekstrand <jason@jlekstrand.net>]

On Tue, May 4, 2021 at 11:13 AM Daniel Vetter <daniel@ffwll.ch> wrote:
>
> On Mon, May 03, 2021 at 10:57:37AM -0500, Jason Ekstrand wrote:
> > The current context uAPI allows for two methods of setting context
> > parameters: SET_CONTEXT_PARAM and CONTEXT_CREATE_EXT_SETPARAM.  The
> > former is allowed to be called at any time while the later happens as
> > part of GEM_CONTEXT_CREATE.  Currently, everything settable via one is
> > settable via the other.  While some params are fairly simple and setting
> > them on a live context is harmless such the context priority, others are
> > far trickier such as the VM or the set of engines.  In order to swap out
> > the VM, for instance, we have to delay until all current in-flight work
> > is complete, swap in the new VM, and then continue.  This leads to a
> > plethora of potential race conditions we'd really rather avoid.
> >
> > Unfortunately, both methods of setting the VM and engine set are in
> > active use today so we can't simply disallow setting the VM or engine
> > set vial SET_CONTEXT_PARAM.  In order to work around this wart, this
> > commit adds a proto-context struct which contains all the context create
> > parameters.
> >
> > Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
>
> Per-patch changelog pls.

Done.

> > ---
> >  drivers/gpu/drm/i915/gem/i915_gem_context.c   | 145 ++++++++++++++----
> >  .../gpu/drm/i915/gem/i915_gem_context_types.h |  22 +++
> >  .../gpu/drm/i915/gem/selftests/mock_context.c |  16 +-
> >  3 files changed, 153 insertions(+), 30 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > index 4835991898ac9..10bd1b6dd1774 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > @@ -191,6 +191,97 @@ static int validate_priority(struct drm_i915_private *i915,
> >       return 0;
> >  }
> >
> > +static void proto_context_close(struct i915_gem_proto_context *pc)
> > +{
> > +     if (pc->vm)
> > +             i915_vm_put(pc->vm);
> > +     kfree(pc);
> > +}
> > +
> > +static int proto_context_set_persistence(struct drm_i915_private *i915,
> > +                                      struct i915_gem_proto_context *pc,
> > +                                      bool persist)
> > +{
> > +     if (persist) {
> > +             /*
> > +              * Only contexts that are short-lived [that will expire or be
> > +              * reset] are allowed to survive past termination. We require
> > +              * hangcheck to ensure that the persistent requests are healthy.
> > +              */
> > +             if (!i915->params.enable_hangcheck)
> > +                     return -EINVAL;
> > +
> > +             __set_bit(UCONTEXT_PERSISTENCE, &pc->user_flags);
>
> Ok so I looked, and the reason __set_bit and friends is for endless
> bitfields, i.e. where user_flags is an actually dynamically sized array.
>
> Given that this is complete overkill I think fully open-coding the bitops
> is the right bikeshed color choice. So

I've fixed it now.  I had fixed it in the last version but it ended up
squashed into the wrong patch. :-(

>                 user_flags &= UCONTEXT_PERSISTENCE;
>
> > +     } else {
> > +             /* To cancel a context we use "preempt-to-idle" */
> > +             if (!(i915->caps.scheduler & I915_SCHEDULER_CAP_PREEMPTION))
> > +                     return -ENODEV;
> > +
> > +             /*
> > +              * If the cancel fails, we then need to reset, cleanly!
> > +              *
> > +              * If the per-engine reset fails, all hope is lost! We resort
> > +              * to a full GPU reset in that unlikely case, but realistically
> > +              * if the engine could not reset, the full reset does not fare
> > +              * much better. The damage has been done.
> > +              *
> > +              * However, if we cannot reset an engine by itself, we cannot
> > +              * cleanup a hanging persistent context without causing
> > +              * colateral damage, and we should not pretend we can by
> > +              * exposing the interface.
> > +              */
> > +             if (!intel_has_reset_engine(&i915->gt))
> > +                     return -ENODEV;
> > +
> > +             __clear_bit(UCONTEXT_PERSISTENCE, &pc->user_flags);
>
>                 user_flags &= ~UCONTEXT_PERSISTENCE;
>
> Similar for all the others.
>
> > +     }
> > +
> > +     return 0;
> > +}
> > +
> > +static struct i915_gem_proto_context *
> > +proto_context_create(struct drm_i915_private *i915, unsigned int flags)
> > +{
> > +     struct i915_gem_proto_context *pc, *err;
> > +
> > +     pc = kzalloc(sizeof(*pc), GFP_KERNEL);
> > +     if (!pc)
> > +             return ERR_PTR(-ENOMEM);
> > +
> > +     if (HAS_FULL_PPGTT(i915)) {
> > +             struct i915_ppgtt *ppgtt;
> > +
> > +             ppgtt = i915_ppgtt_create(&i915->gt);
> > +             if (IS_ERR(ppgtt)) {
> > +                     drm_dbg(&i915->drm, "PPGTT setup failed (%ld)\n",
> > +                             PTR_ERR(ppgtt));
> > +                     err = ERR_CAST(ppgtt);
> > +                     goto proto_close;
> > +             }
> > +             pc->vm = &ppgtt->vm;
>
> I'm not understanding why we're creating the default vm as part of the
> proto context? If we end up setting one this is kinda just wasted
> conditionals, and we're not really gaining in code simplicity. It's just
> whether we create the default vm here (and might need to delete it) or
> when we finalize the context in i915_gem_create_context().
>
> Or am I missing something?
>
> If it's cleaner overall I'm ok with this, just feels a bit silly.

Hrm... I guess there's no particularly good reason.  I think I did it
this way because it seemed like a closer match to the code we had
before.  Also, we need to do it this way if we want GET_CONTEXT_PARAM
with CONTEXT_PARAM_VM to work on a proto-context without finalizing
it.  However, in the end, I'm always finalizing contexts on
GET_CONTEXT_PARAM so that's a non-reason.  I'll see what it looks like
to move it.

> > +     }
> > +
> > +     pc->user_flags = 0;
> > +     __set_bit(UCONTEXT_BANNABLE, &pc->user_flags);
> > +     __set_bit(UCONTEXT_RECOVERABLE, &pc->user_flags);
> > +     proto_context_set_persistence(i915, pc, true);
>
> So this I think is another case of trying too hard to share code, because
> what this essentially does is pick the default through a _very_ contrived
> way:
> - we start out with false since we kzalloc, so everything is cleared
> - we try to enable persistence, but we fail silently, so if the module
>   options don't allow persistence we silently keep it at false

Yeah, it's pretty horrible.  It's a copy+paste of what was there
before but that was terrible too.

> I think open-coding this condition would be a lot better. Also disabling
> hangcheck isn't a thing anymore, but we still have code in it.
>
> I think the right thing here is a patch which removes all occurences of
> params.enable_hangcheck and replaces it with the default (which is true).
>
> And then just set this default here directly, because not doing so would
> randomly break some mesa depending how a modparam is set which otherwise
> has 0 impact. Also I think the other code which checks this could end up
> with a refcount underrun on !execlist platforms.

Yeah, I think this would make things clearer.  Before I go off and do
that, it looks to me like the param still exists.  Maybe we just
ignore it?  Should I delete the whole param?  Just trying to confirm
before I go off and type the wrong code.

--Jason

> > +     pc->sched.priority = I915_PRIORITY_NORMAL;
> > +
> > +     if (flags & I915_CONTEXT_CREATE_FLAGS_SINGLE_TIMELINE) {
> > +             if (!HAS_EXECLISTS(i915)) {
> > +                     err = ERR_PTR(-EINVAL);
> > +                     goto proto_close;
> > +             }
> > +             pc->single_timeline = true;
> > +     }
> > +
> > +     return pc;
> > +
> > +proto_close:
> > +     proto_context_close(pc);
> > +     return err;
> > +}
> > +
> >  static struct i915_address_space *
> >  context_get_vm_rcu(struct i915_gem_context *ctx)
> >  {
> > @@ -660,7 +751,8 @@ static int __context_set_persistence(struct i915_gem_context *ctx, bool state)
> >  }
> >
> >  static struct i915_gem_context *
> > -__create_context(struct drm_i915_private *i915)
> > +__create_context(struct drm_i915_private *i915,
> > +              const struct i915_gem_proto_context *pc)
> >  {
> >       struct i915_gem_context *ctx;
> >       struct i915_gem_engines *e;
> > @@ -673,7 +765,7 @@ __create_context(struct drm_i915_private *i915)
> >
> >       kref_init(&ctx->ref);
> >       ctx->i915 = i915;
> > -     ctx->sched.priority = I915_PRIORITY_NORMAL;
> > +     ctx->sched = pc->sched;
> >       mutex_init(&ctx->mutex);
> >       INIT_LIST_HEAD(&ctx->link);
> >
> > @@ -696,9 +788,7 @@ __create_context(struct drm_i915_private *i915)
> >        * is no remap info, it will be a NOP. */
> >       ctx->remap_slice = ALL_L3_SLICES(i915);
> >
> > -     i915_gem_context_set_bannable(ctx);
> > -     i915_gem_context_set_recoverable(ctx);
> > -     __context_set_persistence(ctx, true /* cgroup hook? */);
> > +     ctx->user_flags = pc->user_flags;
> >
> >       for (i = 0; i < ARRAY_SIZE(ctx->hang_timestamp); i++)
> >               ctx->hang_timestamp[i] = jiffies - CONTEXT_FAST_HANG_JIFFIES;
> > @@ -786,38 +876,23 @@ static void __assign_ppgtt(struct i915_gem_context *ctx,
> >  }
> >
> >  static struct i915_gem_context *
> > -i915_gem_create_context(struct drm_i915_private *i915, unsigned int flags)
> > +i915_gem_create_context(struct drm_i915_private *i915,
> > +                     const struct i915_gem_proto_context *pc)
> >  {
> >       struct i915_gem_context *ctx;
> >       int ret;
> >
> > -     if (flags & I915_CONTEXT_CREATE_FLAGS_SINGLE_TIMELINE &&
> > -         !HAS_EXECLISTS(i915))
> > -             return ERR_PTR(-EINVAL);
> > -
> > -     ctx = __create_context(i915);
> > +     ctx = __create_context(i915, pc);
> >       if (IS_ERR(ctx))
> >               return ctx;
> >
> > -     if (HAS_FULL_PPGTT(i915)) {
> > -             struct i915_ppgtt *ppgtt;
> > -
> > -             ppgtt = i915_ppgtt_create(&i915->gt);
> > -             if (IS_ERR(ppgtt)) {
> > -                     drm_dbg(&i915->drm, "PPGTT setup failed (%ld)\n",
> > -                             PTR_ERR(ppgtt));
> > -                     context_close(ctx);
> > -                     return ERR_CAST(ppgtt);
> > -             }
> > -
> > +     if (pc->vm) {
> >               mutex_lock(&ctx->mutex);
> > -             __assign_ppgtt(ctx, &ppgtt->vm);
> > +             __assign_ppgtt(ctx, pc->vm);
> >               mutex_unlock(&ctx->mutex);
> > -
> > -             i915_vm_put(&ppgtt->vm);
> >       }
> >
> > -     if (flags & I915_CONTEXT_CREATE_FLAGS_SINGLE_TIMELINE) {
> > +     if (pc->single_timeline) {
> >               ret = drm_syncobj_create(&ctx->syncobj,
> >                                        DRM_SYNCOBJ_CREATE_SIGNALED,
> >                                        NULL);
> > @@ -883,6 +958,7 @@ int i915_gem_context_open(struct drm_i915_private *i915,
> >                         struct drm_file *file)
> >  {
> >       struct drm_i915_file_private *file_priv = file->driver_priv;
> > +     struct i915_gem_proto_context *pc;
> >       struct i915_gem_context *ctx;
> >       int err;
> >       u32 id;
> > @@ -892,7 +968,14 @@ int i915_gem_context_open(struct drm_i915_private *i915,
> >       /* 0 reserved for invalid/unassigned ppgtt */
> >       xa_init_flags(&file_priv->vm_xa, XA_FLAGS_ALLOC1);
> >
> > -     ctx = i915_gem_create_context(i915, 0);
> > +     pc = proto_context_create(i915, 0);
> > +     if (IS_ERR(pc)) {
> > +             err = PTR_ERR(pc);
> > +             goto err;
> > +     }
> > +
> > +     ctx = i915_gem_create_context(i915, pc);
> > +     proto_context_close(pc);
> >       if (IS_ERR(ctx)) {
> >               err = PTR_ERR(ctx);
> >               goto err;
> > @@ -1885,6 +1968,7 @@ int i915_gem_context_create_ioctl(struct drm_device *dev, void *data,
> >  {
> >       struct drm_i915_private *i915 = to_i915(dev);
> >       struct drm_i915_gem_context_create_ext *args = data;
> > +     struct i915_gem_proto_context *pc;
> >       struct create_ext ext_data;
> >       int ret;
> >       u32 id;
> > @@ -1907,7 +1991,12 @@ int i915_gem_context_create_ioctl(struct drm_device *dev, void *data,
> >               return -EIO;
> >       }
> >
> > -     ext_data.ctx = i915_gem_create_context(i915, args->flags);
> > +     pc = proto_context_create(i915, args->flags);
> > +     if (IS_ERR(pc))
> > +             return PTR_ERR(pc);
> > +
> > +     ext_data.ctx = i915_gem_create_context(i915, pc);
> > +     proto_context_close(pc);
> >       if (IS_ERR(ext_data.ctx))
> >               return PTR_ERR(ext_data.ctx);
> >
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context_types.h b/drivers/gpu/drm/i915/gem/i915_gem_context_types.h
> > index 5f0673a2129f9..0bf337b6d89ac 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_context_types.h
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_context_types.h
> > @@ -66,6 +66,28 @@ struct i915_gem_engines_iter {
> >       const struct i915_gem_engines *engines;
> >  };
> >
> > +/**
> > + * struct i915_gem_proto_context - prototype context
> > + *
> > + * The struct i915_gem_proto_context represents the creation parameters for
> > + * an i915_gem_context.  This is used to gather parameters provided either
> > + * through creation flags or via SET_CONTEXT_PARAM so that, when we create
> > + * the final i915_gem_context, those parameters can be immutable.
> > + */
> > +struct i915_gem_proto_context {
> > +     /** @vm: See i915_gem_context::vm */
> > +     struct i915_address_space *vm;
> > +
> > +     /** @user_flags: See i915_gem_context::user_flags */
> > +     unsigned long user_flags;
> > +
> > +     /** @sched: See i915_gem_context::sched */
> > +     struct i915_sched_attr sched;
> > +
> > +     /** @single_timeline: See See i915_gem_context::syncobj */
> > +     bool single_timeline;
> > +};
> > +
> >  /**
> >   * struct i915_gem_context - client state
> >   *
> > diff --git a/drivers/gpu/drm/i915/gem/selftests/mock_context.c b/drivers/gpu/drm/i915/gem/selftests/mock_context.c
> > index 51b5a3421b400..e0f512ef7f3c6 100644
> > --- a/drivers/gpu/drm/i915/gem/selftests/mock_context.c
> > +++ b/drivers/gpu/drm/i915/gem/selftests/mock_context.c
> > @@ -80,11 +80,17 @@ void mock_init_contexts(struct drm_i915_private *i915)
> >  struct i915_gem_context *
> >  live_context(struct drm_i915_private *i915, struct file *file)
> >  {
> > +     struct i915_gem_proto_context *pc;
> >       struct i915_gem_context *ctx;
> >       int err;
> >       u32 id;
> >
> > -     ctx = i915_gem_create_context(i915, 0);
> > +     pc = proto_context_create(i915, 0);
> > +     if (IS_ERR(pc))
> > +             return ERR_CAST(pc);
> > +
> > +     ctx = i915_gem_create_context(i915, pc);
> > +     proto_context_close(pc);
> >       if (IS_ERR(ctx))
> >               return ctx;
> >
> > @@ -142,8 +148,14 @@ struct i915_gem_context *
> >  kernel_context(struct drm_i915_private *i915)
> >  {
> >       struct i915_gem_context *ctx;
> > +     struct i915_gem_proto_context *pc;
> > +
> > +     pc = proto_context_create(i915, 0);
> > +     if (IS_ERR(pc))
> > +             return ERR_CAST(pc);
> >
> > -     ctx = i915_gem_create_context(i915, 0);
> > +     ctx = i915_gem_create_context(i915, pc);
> > +     proto_context_close(pc);
>
> I think at large still looks pretty good, but some polish needed.
> -Daniel
>
> >       if (IS_ERR(ctx))
> >               return ctx;
> >
> > --
> > 2.31.1
> >
> > _______________________________________________
> > dri-devel mailing list
> > dri-devel@lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch