Re: [PATCH v2 3/3] mm, swap_cgroup: remove global swap cgroup lock

[Roman Gushchin <roman.gushchin@linux.dev>]

On Tue, Dec 10, 2024 at 05:28:05PM +0800, Kairui Song wrote:
> From: Kairui Song <kasong@tencent.com>
> 
> commit e9e58a4ec3b1 ("memcg: avoid use cmpxchg in swap cgroup maintainance")
> replaced the cmpxchg/xchg with a global irq spinlock because some archs
> doesn't support 2 bytes cmpxchg/xchg. Clearly this won't scale well.
> 
> And as commented in swap_cgroup.c, this lock is not needed for map
> synchronization.
> 
> Emulation of 2 bytes xchg with atomic cmpxchg isn't hard, so implement
> it to get rid of this lock. Introduced two helpers for doing so and they
> can be easily dropped if a generic 2 byte xchg is support.
> 
> Testing using 64G brd and build with build kernel with make -j96 in 1.5G
> memory cgroup using 4k folios showed below improvement (10 test run):
> 
> Before this series:
> Sys time: 10809.46 (stdev 80.831491)
> Real time: 171.41 (stdev 1.239894)
> 
> After this commit:
> Sys time: 9621.26 (stdev 34.620000), -10.42%
> Real time: 160.00 (stdev 0.497814), -6.57%
> 
> With 64k folios and 2G memcg:
> Before this series:
> Sys time: 8231.99 (stdev 30.030994)
> Real time: 143.57 (stdev 0.577394)
> 
> After this commit:
> Sys time: 7403.47 (stdev 6.270000), -10.06%
> Real time: 135.18 (stdev 0.605000), -5.84%
> 
> Sequential swapout of 8G 64k zero folios with madvise (24 test run):
> Before this series:
> 5461409.12 us (stdev 183957.827084)
> 
> After this commit:
> 5420447.26 us (stdev 196419.240317)
> 
> Sequential swapin of 8G 4k zero folios (24 test run):
> Before this series:
> 19736958.916667 us (stdev 189027.246676)
> 
> After this commit:
> 19662182.629630 us (stdev 172717.640614)
> 
> Performance is better or at least not worse for all tests above.
> 
> Signed-off-by: Kairui Song <kasong@tencent.com>

Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>

Thanks!