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    Subject[PATCH v4] proc: Allow pid_revalidate() during LOOKUP_RCU
    Date
    The pid_revalidate() function drops from RCU into REF lookup mode. When
    many threads are resolving paths within /proc in parallel, this can
    result in heavy spinlock contention on d_lockref as each thread tries to
    grab a reference to the /proc dentry (and drop it shortly thereafter).

    Investigation indicates that it is not necessary to drop RCU in
    pid_revalidate(), as no RCU data is modified and the function never
    sleeps. So, remove the LOOKUP_RCU check.

    Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>
    ---
    When running running ~100 parallel instances of "TZ=/etc/localtime ps -fe
    >/dev/null" on a 100CPU machine, the %sys utilization reaches 90%, and perf
    shows the following code path as being responsible for heavy contention on
    the d_lockref spinlock:

    walk_component()
    lookup_fast()
    d_revalidate()
    pid_revalidate() // returns -ECHILD
    unlazy_child()
    lockref_get_not_dead(&nd->path.dentry->d_lockref) <-- contention

    By applying this patch, %sys utilization falls to around 60% under the same
    workload. Although this particular workload is a bit contrived, we have seen
    some monitoring scripts which produced similarly high %sys time due to this
    contention.

    Changes in v4:
    - Simplify by unconditionally calling pid_update_inode() from pid_revalidate,
    and removing the LOOKUP_RCU check.
    Changes in v3:
    - Rather than call pid_update_inode() with flags, create
    proc_inode_needs_update() to determine whether the call can be skipped.
    - Restore the call to the security hook (see next patch).
    Changes in v2:
    - Remove get_pid_task_rcu_user() and get_proc_task_rcu(), since they were
    unnecessary.
    - Remove the call to security_task_to_inode().

    fs/proc/base.c | 15 +++++++--------
    1 file changed, 7 insertions(+), 8 deletions(-)

    diff --git a/fs/proc/base.c b/fs/proc/base.c
    index f52217f432bc..633ef74e8dfd 100644
    --- a/fs/proc/base.c
    +++ b/fs/proc/base.c
    @@ -1974,19 +1974,18 @@ static int pid_revalidate(struct dentry *dentry, unsigned int flags)
    {
    struct inode *inode;
    struct task_struct *task;
    + int ret = 0;

    - if (flags & LOOKUP_RCU)
    - return -ECHILD;
    -
    - inode = d_inode(dentry);
    - task = get_proc_task(inode);
    + rcu_read_lock();
    + inode = d_inode_rcu(dentry);
    + task = pid_task(proc_pid(inode), PIDTYPE_PID);

    if (task) {
    pid_update_inode(task, inode);
    - put_task_struct(task);
    - return 1;
    + ret = 1;
    }
    - return 0;
    + rcu_read_unlock();
    + return ret;
    }

    static inline bool proc_inode_is_dead(struct inode *inode)
    --
    2.25.1
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