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    Subject[PATCH v2] proc: Allow pid_revalidate() during LOOKUP_RCU
    Date
    The pid_revalidate() function requires dropping from RCU into REF lookup
    mode. When many threads are resolving paths within /proc in parallel,
    this can result in heavy spinlock contention as each thread tries to
    grab a reference to the /proc dentry lock (and drop it shortly
    thereafter).

    Allow the pid_revalidate() function to execute under LOOKUP_RCU. When
    updates must be made to the inode due to the owning task performing
    setuid(), drop out of RCU and into REF mode. Remove the call to
    security_task_to_inode(), since we can rely on the call from
    proc_pid_make_inode().

    Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>
    ---
    I'd like to use this patch as an RFC on this approach for reducing spinlock
    contention during many parallel path lookups in the /proc filesystem. The
    contention can be triggered by, for example, running ~100 parallel instances of
    "TZ=/etc/localtime ps -fe >/dev/null" on a 100CPU machine. The %sys utilization
    in such a case reaches around 90%, and profiles show two code paths with high
    utilization:

    walk_component
    lookup_fast
    unlazy_child
    legitimize_root
    __legitimize_path
    lockref_get_not_dead

    terminate_walk
    dput
    dput

    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 high %sys time due to this contention.

    Changes from 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 | 47 +++++++++++++++++++++++++++++++++--------------
    1 file changed, 33 insertions(+), 14 deletions(-)

    diff --git a/fs/proc/base.c b/fs/proc/base.c
    index ebea9501afb8..833d55a59e20 100644
    --- a/fs/proc/base.c
    +++ b/fs/proc/base.c
    @@ -1813,12 +1813,28 @@ int pid_getattr(const struct path *path, struct kstat *stat,
    /*
    * Set <pid>/... inode ownership (can change due to setuid(), etc.)
    */
    -void pid_update_inode(struct task_struct *task, struct inode *inode)
    +static int do_pid_update_inode(struct task_struct *task, struct inode *inode,
    + unsigned int flags)
    {
    - task_dump_owner(task, inode->i_mode, &inode->i_uid, &inode->i_gid);
    + kuid_t uid;
    + kgid_t gid;
    +
    + task_dump_owner(task, inode->i_mode, &uid, &gid);
    + if (uid_eq(uid, inode->i_uid) && gid_eq(gid, inode->i_gid) &&
    + !(inode->i_mode & (S_ISUID | S_ISGID)))
    + return 1;
    + if (flags & LOOKUP_RCU)
    + return -ECHILD;

    + inode->i_uid = uid;
    + inode->i_gid = gid;
    inode->i_mode &= ~(S_ISUID | S_ISGID);
    - security_task_to_inode(task, inode);
    + return 1;
    +}
    +
    +void pid_update_inode(struct task_struct *task, struct inode *inode)
    +{
    + do_pid_update_inode(task, inode, 0);
    }

    /*
    @@ -1830,19 +1846,22 @@ static int pid_revalidate(struct dentry *dentry, unsigned int flags)
    {
    struct inode *inode;
    struct task_struct *task;
    + int rv = 0;

    - if (flags & LOOKUP_RCU)
    - return -ECHILD;
    -
    - inode = d_inode(dentry);
    - task = get_proc_task(inode);
    -
    - if (task) {
    - pid_update_inode(task, inode);
    - put_task_struct(task);
    - return 1;
    + if (flags & LOOKUP_RCU) {
    + inode = d_inode_rcu(dentry);
    + task = pid_task(proc_pid(inode), PIDTYPE_PID);
    + if (task)
    + rv = do_pid_update_inode(task, inode, flags);
    + } else {
    + inode = d_inode(dentry);
    + task = get_proc_task(inode);
    + if (task) {
    + rv = do_pid_update_inode(task, inode, flags);
    + put_task_struct(task);
    + }
    }
    - return 0;
    + return rv;
    }

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