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From:   Radu Rendec <radu.rendec@gmail.com>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Tejun Heo <tj@kernel.org>, Radu Rendec <radu.rendec@gmail.com>
Subject: [PATCH 0/1] kernfs_notify() poll latency
Date:   Thu, 15 Nov 2018 21:09:53 -0500
Message-Id: <20181116020954.24924-1-radu.rendec@gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Hi everyone,

I believe kernfs_notify() poll latency can be improved if the poll
notification is done from kernfs_notify() directly rather than scheduled
work context.

I am sure there are good reasons why the fsnotify notification must be
done from scheduled work context (an obvious one is that it needs to be
able to sleep). But I don't see any reason why the poll notification
could not be done from kernfs_notify(). If there is any, please point it
out - I would highly appreciate it.

I came across this while working on a project that uses the sysfs GPIO
interface to wake a (real time) user space project on GPIO interrupts. I
know that interface is deprecated, but I still believe it's a valid
scenario and may occur with other drivers as well (current or future).

The sysfs GPIO interface (drivers/gpio/gpiolib-sysfs.c) interrupt
handler relies on kernfs_notify() (actually sysfs_notify_dirent(), but
that's just an alias) to wake any thread that may be poll()ing on the
interrupt. It is important to wake the thread as quickly as possible and
going through the kernel worker to handle the scheduled work is much
slower. Since the kernel worker runs with normal priority, this can even
become a case of priority inversion. If a higher priority thread hogs
the CPU, it may delay the kernel worker and in turn the thread that
needs to be notified (which could be a real time thread).

Best regards,
Radu Rendec


Radu Rendec (1):
  Improve kernfs_notify() poll notification latency

 fs/kernfs/file.c | 23 +++++++++++------------
 1 file changed, 11 insertions(+), 12 deletions(-)

-- 
2.17.2