Re: [PATCH v3 2/4] mm/vmap: preload a CPU with one object for split purpose

[Roman Gushchin <guro@fb.com>]

On Wed, May 29, 2019 at 04:27:15PM +0200, Uladzislau Rezki wrote:
> Hello, Roman!
> 
> > On Mon, May 27, 2019 at 11:38:40AM +0200, Uladzislau Rezki (Sony) wrote:
> > > Refactor the NE_FIT_TYPE split case when it comes to an
> > > allocation of one extra object. We need it in order to
> > > build a remaining space.
> > > 
> > > Introduce ne_fit_preload()/ne_fit_preload_end() functions
> > > for preloading one extra vmap_area object to ensure that
> > > we have it available when fit type is NE_FIT_TYPE.
> > > 
> > > The preload is done per CPU in non-atomic context thus with
> > > GFP_KERNEL allocation masks. More permissive parameters can
> > > be beneficial for systems which are suffer from high memory
> > > pressure or low memory condition.
> > > 
> > > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > > ---
> > >  mm/vmalloc.c | 79 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++---
> > >  1 file changed, 76 insertions(+), 3 deletions(-)
> > 
> > Hi Uladzislau!
> > 
> > This patch generally looks good to me (see some nits below),
> > but it would be really great to add some motivation, e.g. numbers.
> > 
> The main goal of this patch to get rid of using GFP_NOWAIT since it is
> more restricted due to allocation from atomic context. IMHO, if we can
> avoid of using it that is a right way to go.
> 
> From the other hand, as i mentioned before i have not seen any issues
> with that on all my test systems during big rework. But it could be
> beneficial for tiny systems where we do not have any swap and are
> limited in memory size.

Ok, that makes sense to me. Is it possible to emulate such a tiny system
on kvm and measure the benefits? Again, not a strong opinion here,
but it will be easier to justify adding a good chunk of code.

> 
> > > 
> > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > index ea1b65fac599..b553047aa05b 100644
> > > --- a/mm/vmalloc.c
> > > +++ b/mm/vmalloc.c
> > > @@ -364,6 +364,13 @@ static LIST_HEAD(free_vmap_area_list);
> > >   */
> > >  static struct rb_root free_vmap_area_root = RB_ROOT;
> > >  
> > > +/*
> > > + * Preload a CPU with one object for "no edge" split case. The
> > > + * aim is to get rid of allocations from the atomic context, thus
> > > + * to use more permissive allocation masks.
> > > + */
> > > +static DEFINE_PER_CPU(struct vmap_area *, ne_fit_preload_node);
> > > +
> > >  static __always_inline unsigned long
> > >  va_size(struct vmap_area *va)
> > >  {
> > > @@ -950,9 +957,24 @@ adjust_va_to_fit_type(struct vmap_area *va,
> > >  		 *   L V  NVA  V R
> > >  		 * |---|-------|---|
> > >  		 */
> > > -		lva = kmem_cache_alloc(vmap_area_cachep, GFP_NOWAIT);
> > > -		if (unlikely(!lva))
> > > -			return -1;
> > > +		lva = __this_cpu_xchg(ne_fit_preload_node, NULL);
> > > +		if (unlikely(!lva)) {
> > > +			/*
> > > +			 * For percpu allocator we do not do any pre-allocation
> > > +			 * and leave it as it is. The reason is it most likely
> > > +			 * never ends up with NE_FIT_TYPE splitting. In case of
> > > +			 * percpu allocations offsets and sizes are aligned to
> > > +			 * fixed align request, i.e. RE_FIT_TYPE and FL_FIT_TYPE
> > > +			 * are its main fitting cases.
> > > +			 *
> > > +			 * There are a few exceptions though, as an example it is
> > > +			 * a first allocation (early boot up) when we have "one"
> > > +			 * big free space that has to be split.
> > > +			 */
> > > +			lva = kmem_cache_alloc(vmap_area_cachep, GFP_NOWAIT);
> > > +			if (!lva)
> > > +				return -1;
> > > +		}
> > >  
> > >  		/*
> > >  		 * Build the remainder.
> > > @@ -1023,6 +1045,48 @@ __alloc_vmap_area(unsigned long size, unsigned long align,
> > >  }
> > >  
> > >  /*
> > > + * Preload this CPU with one extra vmap_area object to ensure
> > > + * that we have it available when fit type of free area is
> > > + * NE_FIT_TYPE.
> > > + *
> > > + * The preload is done in non-atomic context, thus it allows us
> > > + * to use more permissive allocation masks to be more stable under
> > > + * low memory condition and high memory pressure.
> > > + *
> > > + * If success it returns 1 with preemption disabled. In case
> > > + * of error 0 is returned with preemption not disabled. Note it
> > > + * has to be paired with ne_fit_preload_end().
> > > + */
> > > +static int
> > 
> > Cosmetic nit: you don't need a new line here.
> > 
> > > +ne_fit_preload(int nid)
> > 
> I can fix that.
> 
> > > +{
> > > +	preempt_disable();
> > > +
> > > +	if (!__this_cpu_read(ne_fit_preload_node)) {
> > > +		struct vmap_area *node;
> > > +
> > > +		preempt_enable();
> > > +		node = kmem_cache_alloc_node(vmap_area_cachep, GFP_KERNEL, nid);
> > > +		if (node == NULL)
> > > +			return 0;
> > > +
> > > +		preempt_disable();
> > > +
> > > +		if (__this_cpu_cmpxchg(ne_fit_preload_node, NULL, node))
> > > +			kmem_cache_free(vmap_area_cachep, node);
> > > +	}
> > > +
> > > +	return 1;
> > > +}
> > > +
> > > +static void
> > 
> > Here too.
> > 
> > > +ne_fit_preload_end(int preloaded)
> > > +{
> > > +	if (preloaded)
> > > +		preempt_enable();
> > > +}
> I can fix that.
> 
> > 
> > I'd open code it. It's used only once, but hiding preempt_disable()
> > behind a helper makes it harder to understand and easier to mess.
> > 
> > Then ne_fit_preload() might require disabled preemption (which it can
> > temporarily re-enable), so that preempt_enable()/disable() logic
> > will be in one place.
> > 
> I see your point. One of the aim was to make less clogged the
> alloc_vmap_area() function. But we can refactor it like you say:
> 
> <snip>
>  static void
> @@ -1091,7 +1089,7 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
>                                 unsigned long vstart, unsigned long vend,
>                                 int node, gfp_t gfp_mask)
>  {
> -       struct vmap_area *va;
> +       struct vmap_area *va, *pva;
>         unsigned long addr;
>         int purged = 0;
>         int preloaded;
> @@ -1122,16 +1120,26 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
>          * Just proceed as it is. "overflow" path will refill
>          * the cache we allocate from.
>          */
> -       ne_fit_preload(&preloaded);
> +       preempt_disable();
> +       if (!__this_cpu_read(ne_fit_preload_node)) {
> +               preempt_enable();
> +               pva = kmem_cache_alloc_node(vmap_area_cachep, GFP_KERNEL, node);
> +               preempt_disable();
> +
> +               if (__this_cpu_cmpxchg(ne_fit_preload_node, NULL, pva)) {
> +                       if (pva)
> +                               kmem_cache_free(vmap_area_cachep, pva);
> +               }
> +       }
> +
>         spin_lock(&vmap_area_lock);
> +       preempt_enable();
>  
>         /*
>          * If an allocation fails, the "vend" address is
>          * returned. Therefore trigger the overflow path.
>          */
>         addr = __alloc_vmap_area(size, align, vstart, vend);
> -       ne_fit_preload_end(preloaded);
> -
>         if (unlikely(addr == vend))
>                 goto overflow;
> <snip>
> 
> Do you mean something like that? If so, i can go with that, unless there are no
> any objections from others.

Yes, it looks much better to me!

Thank you!