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

[Uladzislau Rezki]

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@xxxxxxxxx>
> > ---
> >  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.

> > 
> > 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.

Thank you for your comments, Roman!

--
Vlad Rezki