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SubjectRe: [PATCH v3 0/4] mm/slub: Fix count_partial() problem

On Mon, Mar 15, 2021 at 07:49:57PM +0100, Vlastimil Babka wrote:
> On 3/9/21 4:25 PM, Xunlei Pang wrote:
> > count_partial() can hold n->list_lock spinlock for quite long, which
> > makes much trouble to the system. This series eliminate this problem.
>
> Before I check the details, I have two high-level comments:
>
> - patch 1 introduces some counting scheme that patch 4 then changes, could we do
> this in one step to avoid the churn?
>
> - the series addresses the concern that spinlock is being held, but doesn't
> address the fact that counting partial per-node slabs is not nearly enough if we
> want accurate <active_objs> in /proc/slabinfo because there are also percpu
> slabs and per-cpu partial slabs, where we don't track the free objects at all.
> So after this series while the readers of /proc/slabinfo won't block the
> spinlock, they will get the same garbage data as before. So Christoph is not
> wrong to say that we can just report active_objs == num_objs and it won't
> actually break any ABI.
> At the same time somebody might actually want accurate object statistics at the
> expense of peak performance, and it would be nice to give them such option in
> SLUB. Right now we don't provide this accuracy even with CONFIG_SLUB_STATS,
> although that option provides many additional tuning stats, with additional
> overhead.
> So my proposal would be a new config for "accurate active objects" (or just tie
> it to CONFIG_SLUB_DEBUG?) that would extend the approach of percpu counters in
> patch 4 to all alloc/free, so that it includes percpu slabs. Without this config
> enabled, let's just report active_objs == num_objs.

It sounds really good to me! The only thing, I'd avoid introducing a new option
and use CONFIG_SLUB_STATS instead.

It seems like CONFIG_SLUB_DEBUG is a more popular option than CONFIG_SLUB_STATS.
CONFIG_SLUB_DEBUG is enabled on my Fedora workstation, CONFIG_SLUB_STATS is off.
I doubt an average user needs this data, so I'd go with CONFIG_SLUB_STATS.

Thanks!

>
> Vlastimil
>
> > v1->v2:
> > - Improved changelog and variable naming for PATCH 1~2.
> > - PATCH3 adds per-cpu counter to avoid performance regression
> > in concurrent __slab_free().
> >
> > v2->v3:
> > - Changed "page->inuse" to the safe "new.inuse", etc.
> > - Used CONFIG_SLUB_DEBUG and CONFIG_SYSFS condition for new counters.
> > - atomic_long_t -> unsigned long
> >
> > [Testing]
> > There seems might be a little performance impact under extreme
> > __slab_free() concurrent calls according to my tests.
> >
> > On my 32-cpu 2-socket physical machine:
> > Intel(R) Xeon(R) CPU E5-2650 v2 @ 2.60GHz
> >
> > 1) perf stat --null --repeat 10 -- hackbench 20 thread 20000
> >
> > == original, no patched
> > Performance counter stats for 'hackbench 20 thread 20000' (10 runs):
> >
> > 24.536050899 seconds time elapsed ( +- 0.24% )
> >
> >
> > Performance counter stats for 'hackbench 20 thread 20000' (10 runs):
> >
> > 24.588049142 seconds time elapsed ( +- 0.35% )
> >
> >
> > == patched with patch1~4
> > Performance counter stats for 'hackbench 20 thread 20000' (10 runs):
> >
> > 24.670892273 seconds time elapsed ( +- 0.29% )
> >
> >
> > Performance counter stats for 'hackbench 20 thread 20000' (10 runs):
> >
> > 24.746755689 seconds time elapsed ( +- 0.21% )
> >
> >
> > 2) perf stat --null --repeat 10 -- hackbench 32 thread 20000
> >
> > == original, no patched
> > Performance counter stats for 'hackbench 32 thread 20000' (10 runs):
> >
> > 39.784911855 seconds time elapsed ( +- 0.14% )
> >
> > Performance counter stats for 'hackbench 32 thread 20000' (10 runs):
> >
> > 39.868687608 seconds time elapsed ( +- 0.19% )
> >
> > == patched with patch1~4
> > Performance counter stats for 'hackbench 32 thread 20000' (10 runs):
> >
> > 39.681273015 seconds time elapsed ( +- 0.21% )
> >
> > Performance counter stats for 'hackbench 32 thread 20000' (10 runs):
> >
> > 39.681238459 seconds time elapsed ( +- 0.09% )
> >
> >
> > Xunlei Pang (4):
> > mm/slub: Introduce two counters for partial objects
> > mm/slub: Get rid of count_partial()
> > percpu: Export per_cpu_sum()
> > mm/slub: Use percpu partial free counter
> >
> > include/linux/percpu-defs.h | 10 ++++
> > kernel/locking/percpu-rwsem.c | 10 ----
> > mm/slab.h | 4 ++
> > mm/slub.c | 120 +++++++++++++++++++++++++++++-------------
> > 4 files changed, 97 insertions(+), 47 deletions(-)
> >
>

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