Re: [PATCH 4/4] mm, swap_cgroup: remove global swap cgroup lock

Yosry Ahmed
2024-12-02T20:36:35Z

On Mon, Dec 2, 2024 at 11:28 AM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> On Mon, Dec 2, 2024 at 10:42 AM Kairui Song <ryncsn@gmail.com> 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 cmpxchg/xchg with atomic isn't hard, so implement
> > it to get rid of this lock.
> >
> > 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: 10730.08 (stdev 49.030728)
> > Real time: 171.03 (stdev 0.850355)
> >
> > After this commit:
> > Sys time: 9612.24 (stdev 66.310789), -10.42%
> > Real time: 159.78 (stdev 0.577193), -6.57%
> >
> > With 64k folios and 2G memcg:
> > Before this series:
> > Sys time: 7626.77 (stdev 43.545517)
> > Real time: 136.22 (stdev 1.265544)
> >
> > After this commit:
> > Sys time: 6936.03 (stdev 39.996280), -9.06%
> > Real time: 129.65 (stdev 0.880039), -4.82%
> >
> > Sequential swapout of 8G 4k zero folios (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>
> > ---
> > mm/swap_cgroup.c | 56 +++++++++++++++++++++++++++++++++++-------------
> > 1 file changed, 41 insertions(+), 15 deletions(-)
> >
> > diff --git a/mm/swap_cgroup.c b/mm/swap_cgroup.c
> > index a76afdc3666a..028f5e6be3f0 100644
> > --- a/mm/swap_cgroup.c
> > +++ b/mm/swap_cgroup.c
> > @@ -5,6 +5,15 @@
> >
> > #include <linux/swapops.h> /* depends on mm.h include */
> >
> > +#define ID_PER_UNIT (sizeof(atomic_t) / sizeof(unsigned short))
> > +struct swap_cgroup_unit {
> > + union {
> > + int raw;
> > + atomic_t val;
> > + unsigned short __id[ID_PER_UNIT];
> > + };
> > +};
>
> This doubles the size of the per-entry data, right?

Oh we don't, we just store 2 ids in an int instead of storing each id
individually. But the question below still stands, can't we just use
cmpxchg() directly on the id?

>
> Why do we need this? I thought cmpxchg() supports multiple sizes and
> will already do the emulation for us.