Messages in this thread Patch in this message | | | Date | Thu, 2 Mar 2023 06:27:41 +0000 | Subject | [PATCH] pstore: Revert pmsg_lock back to a normal mutex | From | John Stultz <> |
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This reverts commit 76d62f24db07f22ccf9bc18ca793c27d4ebef721.
So while priority inversion on the pmsg_lock is an occasional problem that an rt_mutex would help with, in uses where logging is writing to pmsg heavily from multiple threads, the pmsg_lock can be heavily contended.
Normal mutexes can do adaptive spinning, which keeps the contention overhead fairly low maybe adding on the order of 10s of us delay waiting, but the slowpath w/ rt_mutexes makes the blocked tasks sleep & wake. This makes matters worse when there is heavy contentention, as it just allows additional threads to run and line up to try to take the lock.
It devolves to a worse case senerio where the lock acquisition and scheduling overhead dominates, and each thread is waiting on the order of ~ms to do ~us of work.
Obviously, having tons of threads all contending on a single lock for logging is non-optimal, so the proper fix is probably reworking pstore pmsg to have per-cpu buffers so we don't have contention.
But in the short term, lets revert the change to the rt_mutex and go back to normal mutexes to avoid a potentially major performance regression.
Cc: Wei Wang <wvw@google.com> Cc: Midas Chien<midaschieh@google.com> Cc: Chunhui Li (李春辉)" <chunhui.li@mediatek.com> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Kees Cook <keescook@chromium.org> Cc: Anton Vorontsov <anton@enomsg.org> Cc: "Guilherme G. Piccoli" <gpiccoli@igalia.com> Cc: Tony Luck <tony.luck@intel.com> Cc: kernel-team@android.com Fixes: 76d62f24db07 ("pstore: Switch pmsg_lock to an rt_mutex to avoid priority inversion") Reported-by: Chunhui Li (李春辉)" <chunhui.li@mediatek.com> Signed-off-by: John Stultz <jstultz@google.com> --- fs/pstore/pmsg.c | 7 +++---- 1 file changed, 3 insertions(+), 4 deletions(-)
diff --git a/fs/pstore/pmsg.c b/fs/pstore/pmsg.c index ab82e5f05346..b31c9c72d90b 100644 --- a/fs/pstore/pmsg.c +++ b/fs/pstore/pmsg.c @@ -7,10 +7,9 @@ #include <linux/device.h> #include <linux/fs.h> #include <linux/uaccess.h> -#include <linux/rtmutex.h> #include "internal.h" -static DEFINE_RT_MUTEX(pmsg_lock); +static DEFINE_MUTEX(pmsg_lock); static ssize_t write_pmsg(struct file *file, const char __user *buf, size_t count, loff_t *ppos) @@ -29,9 +28,9 @@ static ssize_t write_pmsg(struct file *file, const char __user *buf, if (!access_ok(buf, count)) return -EFAULT; - rt_mutex_lock(&pmsg_lock); + mutex_lock(&pmsg_lock); ret = psinfo->write_user(&record, buf); - rt_mutex_unlock(&pmsg_lock); + mutex_unlock(&pmsg_lock); return ret ? ret : count; } -- 2.39.2.722.g9855ee24e9-goog
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