Re: [PATCH 3/5] mm/memcg: Cache vmstat data in percpu memcg_stock_pcp

[Waiman Long]

On 4/12/21 2:22 PM, Roman Gushchin wrote:
> On Fri, Apr 09, 2021 at 07:18:40PM -0400, Waiman Long wrote:
> > Before the new slab memory controller with per object byte charging,
> > charging and vmstat data update happen only when new slab pages are
> > allocated or freed. Now they are done with every kmem_cache_alloc()
> > and kmem_cache_free(). This causes additional overhead for workloads
> > that generate a lot of alloc and free calls.
> >
> > The memcg_stock_pcp is used to cache byte charge for a specific
> > obj_cgroup to reduce that overhead. To further reducing it, this patch
> > makes the vmstat data cached in the memcg_stock_pcp structure as well
> > until it accumulates a page size worth of update or when other cached
> > data change.
> The idea makes total sense to me and also gives a hope to remove
> byte-sized vmstats in the long-term.
>
> > On a 2-socket Cascade Lake server with instrumentation enabled and this
> > patch applied, it was found that about 17% (946796 out of 5515184) of the
> > time when __mod_obj_stock_state() is called leads to an actual call to
> > mod_objcg_state() after initial boot. When doing parallel kernel build,
> > the figure was about 16% (21894614 out of 139780628). So caching the
> > vmstat data reduces the number of calls to mod_objcg_state() by more
> > than 80%.
> >
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > ---
> >   mm/memcontrol.c | 78 +++++++++++++++++++++++++++++++++++++++++++------
> >   mm/slab.h       | 26 +++++++----------
> >   2 files changed, 79 insertions(+), 25 deletions(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index b19100c68aa0..539c3b632e47 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -2220,7 +2220,10 @@ struct memcg_stock_pcp {
> >   
> >   #ifdef CONFIG_MEMCG_KMEM
> >   	struct obj_cgroup *cached_objcg;
> > +	struct pglist_data *cached_pgdat;
> >   	unsigned int nr_bytes;
> > +	int vmstat_idx;
> > +	int vmstat_bytes;
> >   #endif
> Because vmstat_idx can realistically take only 3 values (slab_reclaimable,
> slab_unreclaimable and percpu), I wonder if it's better to have
> vmstat_bytes[3] and save a bit more on the reduced number of flushes?
> It must be an often case when a complex (reclaimable) kernel object has
> non-reclaimable parts (e.g. kmallocs) or percpu counters.
> If the difference will be too small, maybe the current form is better.

I have thought about that too. However, that will make the code more 
complex. So I decided to cache just one for now. We can certainly play 
around with caching more in a later patch.

Cheers,
Longman