Re: [Intel-gfx] [PATCH 01/11] drm/i915: Release i915_gem_context from a worker

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 02/09/2021 21:02, Daniel Vetter wrote:
> On Thu, Sep 2, 2021 at 6:20 PM Tvrtko Ursulin
> <tvrtko.ursulin@linux.intel.com> wrote:
>> On 02/09/2021 16:05, Daniel Vetter wrote:
>>> On Thu, Sep 2, 2021 at 2:42 PM Tvrtko Ursulin
>>> <tvrtko.ursulin@linux.intel.com> wrote:
>>>>
>>>>
>>>> On 13/08/2021 21:30, Daniel Vetter wrote:
>>>>> The only reason for this really is the i915_gem_engines->fence
>>>>> callback engines_notify(), which exists purely as a fairly funky
>>>>> reference counting scheme for that. Otherwise all other callers are
>>>>> from process context, and generally fairly benign locking context.
>>>>
>>>> There is reset which definitely isn't process context.
>>>
>>> gpu reset runs in process context. The tasklet context is the
>>> engines_notify I'm talking about above.
>>
>> I haven't looked very deeply but please double check the path from
>> execlists_submission_tasklet -> execlists_reset -> intel_engine_reset ->
>> __intel_engine_reset -> execlists_reset_rewind -> execlists_reset_csb ->
>> execlists_reset_active -> __i915_request_reset -> mark_guilty ->
>> i915_gem_context_put.
> 
> Thanks for pointing this out, I'll add it to the commit message.
> 
> More stuff to fix, yay.
> 
>>>> Otherwise I did not really get from the commit message is this patch
>>>> fixing an existing problem or preparing something for the future. If the
>>>> former then as I wrote above - I am pretty sure there are call sites
>>>> from the tasklet already.
>>>>
>>>> Regards,
>>>>
>>>> Tvrtko
>>>>
>>>>> Unfortunately untangling that requires some major surgery, and we have
>>>>> a few i915_gem_context reference counting bugs that need fixing, and
>>>>> they blow in the current hardirq calling context, so we need a
>>>>> stop-gap measure.
>>>
>>> I guess this para wasn't clear, but subsequent patches fix the
>>> refcount bugs and need this prep patch here.
>>
>> So up to where in the series are those fixes and where other stuff
>> follows? Worth spliting and having cover letters perhaps? Is the fixing
>> part applicable to the existing code or only comes to play with the
>> syncobj single timeline changes?
> 
> There's Fixes: lines. One is timeline syncobj, the other is 2 years old.

So first two patches are standalone and fix the immediate bug? Could you 
describe the composition and doings of the series in a cover letter so 
it's possible to have an overview of chunk of work tackled?

Regards,

Tvrtko

> -Daniel
> 
>>
>> Regards,
>>
>> Tvrtko
>>
>>> -Daniel
>>>
>>>>>
>>>>> Put a FIXME comment in when this should be removable again.
>>>>>
>>>>> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
>>>>> Cc: Jon Bloomfield <jon.bloomfield@intel.com>
>>>>> Cc: Chris Wilson <chris@chris-wilson.co.uk>
>>>>> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
>>>>> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
>>>>> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
>>>>> Cc: "Thomas Hellström" <thomas.hellstrom@linux.intel.com>
>>>>> Cc: Matthew Auld <matthew.auld@intel.com>
>>>>> Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
>>>>> Cc: Dave Airlie <airlied@redhat.com>
>>>>> Cc: Jason Ekstrand <jason@jlekstrand.net>
>>>>> ---
>>>>>     drivers/gpu/drm/i915/gem/i915_gem_context.c       | 13 +++++++++++--
>>>>>     drivers/gpu/drm/i915/gem/i915_gem_context_types.h | 12 ++++++++++++
>>>>>     2 files changed, 23 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
>>>>> index fd169cf2f75a..051bc357ff65 100644
>>>>> --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
>>>>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
>>>>> @@ -986,9 +986,10 @@ static struct i915_gem_engines *user_engines(struct i915_gem_context *ctx,
>>>>>         return err;
>>>>>     }
>>>>>
>>>>> -void i915_gem_context_release(struct kref *ref)
>>>>> +static void i915_gem_context_release_work(struct work_struct *work)
>>>>>     {
>>>>> -     struct i915_gem_context *ctx = container_of(ref, typeof(*ctx), ref);
>>>>> +     struct i915_gem_context *ctx = container_of(work, typeof(*ctx),
>>>>> +                                                 release_work);
>>>>>
>>>>>         trace_i915_context_free(ctx);
>>>>>         GEM_BUG_ON(!i915_gem_context_is_closed(ctx));
>>>>> @@ -1002,6 +1003,13 @@ void i915_gem_context_release(struct kref *ref)
>>>>>         kfree_rcu(ctx, rcu);
>>>>>     }
>>>>>
>>>>> +void i915_gem_context_release(struct kref *ref)
>>>>> +{
>>>>> +     struct i915_gem_context *ctx = container_of(ref, typeof(*ctx), ref);
>>>>> +
>>>>> +     queue_work(ctx->i915->wq, &ctx->release_work);
>>>>> +}
>>>>> +
>>>>>     static inline struct i915_gem_engines *
>>>>>     __context_engines_static(const struct i915_gem_context *ctx)
>>>>>     {
>>>>> @@ -1303,6 +1311,7 @@ i915_gem_create_context(struct drm_i915_private *i915,
>>>>>         ctx->sched = pc->sched;
>>>>>         mutex_init(&ctx->mutex);
>>>>>         INIT_LIST_HEAD(&ctx->link);
>>>>> +     INIT_WORK(&ctx->release_work, i915_gem_context_release_work);
>>>>>
>>>>>         spin_lock_init(&ctx->stale.lock);
>>>>>         INIT_LIST_HEAD(&ctx->stale.engines);
>>>>> 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 94c03a97cb77..0c38789bd4a8 100644
>>>>> --- a/drivers/gpu/drm/i915/gem/i915_gem_context_types.h
>>>>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_context_types.h
>>>>> @@ -288,6 +288,18 @@ struct i915_gem_context {
>>>>>          */
>>>>>         struct kref ref;
>>>>>
>>>>> +     /**
>>>>> +      * @release_work:
>>>>> +      *
>>>>> +      * Work item for deferred cleanup, since i915_gem_context_put() tends to
>>>>> +      * be called from hardirq context.
>>>>> +      *
>>>>> +      * FIXME: The only real reason for this is &i915_gem_engines.fence, all
>>>>> +      * other callers are from process context and need at most some mild
>>>>> +      * shuffling to pull the i915_gem_context_put() call out of a spinlock.
>>>>> +      */
>>>>> +     struct work_struct release_work;
>>>>> +
>>>>>         /**
>>>>>          * @rcu: rcu_head for deferred freeing.
>>>>>          */
>>>>>
>>>
>>>
>>>
> 
> 
>