On 3/7/23 07:56, Qi Zheng wrote:
> Like global slab shrink, this commit also uses SRCU to make
> memcg slab shrink lockless.
>
> We can reproduce the down_read_trylock() hotspot through the
> following script:
>
> ```
>
> DIR="/root/shrinker/memcg/mnt"
>
> do_create()
> {
> mkdir -p /sys/fs/cgroup/memory/test
> mkdir -p /sys/fs/cgroup/perf_event/test
> echo 4G > /sys/fs/cgroup/memory/test/memory.limit_in_bytes
> for i in `seq 0 $1`;
> do
> mkdir -p /sys/fs/cgroup/memory/test/$i;
> echo $$ > /sys/fs/cgroup/memory/test/$i/cgroup.procs;
> echo $$ > /sys/fs/cgroup/perf_event/test/cgroup.procs;
> mkdir -p $DIR/$i;
> done
> }
>
> do_mount()
> {
> for i in `seq $1 $2`;
> do
> mount -t tmpfs $i $DIR/$i;
> done
> }
>
> do_touch()
> {
> for i in `seq $1 $2`;
> do
> echo $$ > /sys/fs/cgroup/memory/test/$i/cgroup.procs;
> echo $$ > /sys/fs/cgroup/perf_event/test/cgroup.procs;
> dd if=/dev/zero of=$DIR/$i/file$i bs=1M count=1 &
> done
> }
>
> case "$1" in
> touch)
> do_touch $2 $3
> ;;
> test)
> do_create 4000
> do_mount 0 4000
> do_touch 0 3000
> ;;
> *)
> exit 1
> ;;
> esac
> ```
>
> Save the above script, then run test and touch commands.
> Then we can use the following perf command to view hotspots:
>
> perf top -U -F 999
>
> 1) Before applying this patchset:
>
> 32.31% [kernel] [k] down_read_trylock
> 19.40% [kernel] [k] pv_native_safe_halt
> 16.24% [kernel] [k] up_read
> 15.70% [kernel] [k] shrink_slab
> 4.69% [kernel] [k] _find_next_bit
> 2.62% [kernel] [k] shrink_node
> 1.78% [kernel] [k] shrink_lruvec
> 0.76% [kernel] [k] do_shrink_slab
>
> 2) After applying this patchset:
>
> 27.83% [kernel] [k] _find_next_bit
> 16.97% [kernel] [k] shrink_slab
> 15.82% [kernel] [k] pv_native_safe_halt
> 9.58% [kernel] [k] shrink_node
> 8.31% [kernel] [k] shrink_lruvec
> 5.64% [kernel] [k] do_shrink_slab
> 3.88% [kernel] [k] mem_cgroup_iter
>
> At the same time, we use the following perf command to capture
> IPC information:
>
> perf stat -e cycles,instructions -G test -a --repeat 5 -- sleep 10
>
> 1) Before applying this patchset:
>
> Performance counter stats for 'system wide' (5 runs):
>
> 454187219766 cycles test ( +- 1.84% )
> 78896433101 instructions test # 0.17 insn per cycle ( +- 0.44% )
>
> 10.0020430 +- 0.0000366 seconds time elapsed ( +- 0.00% )
>
> 2) After applying this patchset:
>
> Performance counter stats for 'system wide' (5 runs):
>
> 841954709443 cycles test ( +- 15.80% ) (98.69%)
> 527258677936 instructions test # 0.63 insn per cycle ( +- 15.11% ) (98.68%)
>
> 10.01064 +- 0.00831 seconds time elapsed ( +- 0.08% )
>
> We can see that IPC drops very seriously when calling
> down_read_trylock() at high frequency. After using SRCU,
> the IPC is at a normal level.
The interpretation looks somewhat weird to me. I'd say the workload is
stalled a lot as it fails the trylock (there might be some optimistic
spinning perhaps) and then goes to sleep. See how "pv_native_safe_halt" is
also more prominent in before. And because of that sleeping, there's less
instructions executed in the same amount of cycles (as it's a system wide
collection, otherwise it wouldn't be collecting the sleeping processes).
>
> Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
Other than that:
Acked-by: Vlastimil Babka <Vbabka@xxxxxxx>
A small thing below:
> ---
> mm/vmscan.c | 46 +++++++++++++++++++++++++++-------------------
> 1 file changed, 27 insertions(+), 19 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 8515ac40bcaf..1de9bc3e5aa2 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -57,6 +57,7 @@
> #include <linux/khugepaged.h>
> #include <linux/rculist_nulls.h>
> #include <linux/random.h>
> +#include <linux/srcu.h>
I guess this should have been in patch 2/8 already? It may work accidentaly
because some other header pulls it transitively...
