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(-) > > >
