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    SubjectRe: [PATCH 3/3] mm: Count list_lru_one::nr_items lockless
    On Tue, Aug 22, 2017 at 03:29:35PM +0300, Kirill Tkhai wrote:
    > During the reclaiming slab of a memcg, shrink_slab iterates
    > over all registered shrinkers in the system, and tries to count
    > and consume objects related to the cgroup. In case of memory
    > pressure, this behaves bad: I observe high system time and
    > time spent in list_lru_count_one() for many processes on RHEL7
    > kernel (collected via $perf record --call-graph fp -j k -a):
    >
    > 0,50% nixstatsagent [kernel.vmlinux] [k] _raw_spin_lock [k] _raw_spin_lock
    > 0,26% nixstatsagent [kernel.vmlinux] [k] shrink_slab [k] shrink_slab
    > 0,23% nixstatsagent [kernel.vmlinux] [k] super_cache_count [k] super_cache_count
    > 0,15% nixstatsagent [kernel.vmlinux] [k] __list_lru_count_one.isra.2 [k] _raw_spin_lock
    > 0,15% nixstatsagent [kernel.vmlinux] [k] list_lru_count_one [k] __list_lru_count_one.isra.2
    >
    > 0,94% mysqld [kernel.vmlinux] [k] _raw_spin_lock [k] _raw_spin_lock
    > 0,57% mysqld [kernel.vmlinux] [k] shrink_slab [k] shrink_slab
    > 0,51% mysqld [kernel.vmlinux] [k] super_cache_count [k] super_cache_count
    > 0,32% mysqld [kernel.vmlinux] [k] __list_lru_count_one.isra.2 [k] _raw_spin_lock
    > 0,32% mysqld [kernel.vmlinux] [k] list_lru_count_one [k] __list_lru_count_one.isra.2
    >
    > 0,73% sshd [kernel.vmlinux] [k] _raw_spin_lock [k] _raw_spin_lock
    > 0,35% sshd [kernel.vmlinux] [k] shrink_slab [k] shrink_slab
    > 0,32% sshd [kernel.vmlinux] [k] super_cache_count [k] super_cache_count
    > 0,21% sshd [kernel.vmlinux] [k] __list_lru_count_one.isra.2 [k] _raw_spin_lock
    > 0,21% sshd [kernel.vmlinux] [k] list_lru_count_one [k] __list_lru_count_one.isra.2

    It would be nice to see how this is improved by this patch.
    Can you try to record the traces on the vanilla kernel with
    and without this patch?

    >
    > This patch aims to make super_cache_count() more effective. It
    > makes __list_lru_count_one() count nr_items lockless to minimize
    > overhead introducing by locking operation, and to make parallel
    > reclaims more scalable.
    >
    > The lock won't be taken on shrinker::count_objects(),
    > it would be taken only for the real shrink by the thread,
    > who realizes it.
    >

    > https://jira.sw.ru/browse/PSBM-69296

    Not relevant.

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