From: John Stultz jstultz@google.com
[ Upstream commit 5239a89b06d6b199f133bf0ffea421683187f257 ]
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.
After this change landed, it was reported that cases where the mutex locking overhead was commonly adding on the order of 10s of usecs delay had suddenly jumped to ~msec delay with rtmutex.
It seems the slight differences in the locks under this level of contention causes the normal mutexes to utilize the spinning optimizations, while the rtmutexes end up in the sleeping slowpath (which allows additional threads to pile on trying to take the lock).
In this case, 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.
Additionally, Steven Rostedt has provided some furhter optimizations for rtmutexes that improves the rtmutex spinning path, but at least in my testing, I still see the test tripping into the sleeping path on rtmutexes while utilizing the spinning path with mutexes.
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. And we can work on optimizations to both rtmutexes and finer-grained locking for pstore pmsg in the future.
Cc: Wei Wang wvw@google.com Cc: Midas Chienmidaschieh@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 Signed-off-by: Kees Cook keescook@chromium.org Link: https://lore.kernel.org/r/20230308204043.2061631-1-jstultz@google.com Signed-off-by: Sasha Levin sashal@kernel.org --- 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 ab82e5f053464..b31c9c72d90b4 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; }