On 3/16/21 2:49 AM, 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. If maintainers don't mind this inaccuracy which I also doubt its importance, then it becomes easy. For fear that some people who really cares, introducing an extra config(default-off) for it would be a good option. > 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. For percpu slabs, the numbers can be retrieved from the existing slub_percpu_partial()->pobjects, looks no need extra work. > > 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(-) >>
