linux-kernel - Re: [PATCH] mm/slub: improve count_partial() for CONFIG_SLUB_CPU

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Message-ID: <20200227183519.GA50628@carbon.dhcp.thefacebook.com>
Date:   Thu, 27 Feb 2020 10:35:19 -0800
From:   Roman Gushchin <guro@...com>
To:     Christopher Lameter <cl@...ux.com>
CC:     Wen Yang <wenyang@...ux.alibaba.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Xunlei Pang <xlpang@...ux.alibaba.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/slub: improve count_partial() for
 CONFIG_SLUB_CPU_PARTIAL

On Wed, Feb 26, 2020 at 06:31:28PM +0000, Christoph Lameter wrote:
> On Mon, 24 Feb 2020, Roman Gushchin wrote:
> 
> > > I suggest that you simply use the number of partial slabs and multiply
> > > them by the number of objects in a slab and use that as a value. Both
> > > values are readily available via /sys/kernel/slab/<...>/
> >
> > So maybe something like this?
> >
> > @@ -5907,7 +5907,9 @@ void get_slabinfo(struct kmem_cache *s, struct slabinfo *sinfo)
> >  	for_each_kmem_cache_node(s, node, n) {
> >  		nr_slabs += node_nr_slabs(n);
> >  		nr_objs += node_nr_objs(n);
> > +#ifndef CONFIG_SLUB_CPU_PARTIAL
> >  		nr_free += count_partial(n, count_free);
> > +#endif
> >  	}
> 
> Why would not having cpu partials screws up the counting of objects in
> partial slabs?
> 
> 
> You dont need kernel mods for this. The numbers are exposed already in
> /sys/kernel/slab/xxx.

Stepping a little bit back, there are two independent problems:

1) Reading /proc/slabinfo can cause latency spikes in concurrent memory allocations.
   This is the problem which Wen raised initially.

2) The number of active objects as displayed in /proc/slabinfo is misleadingly
   big if CONFIG_SLUB_CPU_PARTIAL is set.

Maybe mixing them in a single workaround wasn't the best idea, but what do you
suggest instead?

Thank you!

Roman