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

[Yang Shi <shy828301@gmail.com>]

On Mon, Mar 15, 2021 at 12:15 PM Roman Gushchin <guro@fb.com> wrote:
>
>
> 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.

I think CONFIG_SLUB_DEBUG is enabled by default on most distros since
it is supposed not incur too much overhead unless specific debug (i.e.
red_zone) is turned on on demand.

>
> 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(-)
> > >
> >
>