linux-kernel - Re: [PATCH] gpiolib: fix the speed of descriptor label setting with SRCU

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Message-ID: <CAMRc=MexihBpLBcY-8aX06buUYmtE07ZpkMq0Ho3jrHb6VE7Sw@mail.gmail.com>
Date: Tue, 7 May 2024 16:48:04 +0200
From: Bartosz Golaszewski <brgl@...ev.pl>
To: paulmck@...nel.org
Cc: Kent Gibson <warthog618@...il.com>, Linus Walleij <linus.walleij@...aro.org>, 
	linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>, Neil Armstrong <neil.armstrong@...aro.org>
Subject: Re: [PATCH] gpiolib: fix the speed of descriptor label setting with SRCU

On Tue, May 7, 2024 at 4:24 PM Paul E. McKenney <paulmck@...nel.org> wrote:
>
> On Tue, May 07, 2024 at 02:13:46PM +0200, Bartosz Golaszewski wrote:
> > From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> >
> > Commit 1f2bcb8c8ccd ("gpio: protect the descriptor label with SRCU")
> > caused a massive drop in performance of requesting GPIO lines due to the
> > call to synchronize_srcu() on each label change. Rework the code to not
> > wait until all read-only users are done with reading the label but
> > instead atomically replace the label pointer and schedule its release
> > after all read-only critical sections are done.
> >
> > To that end wrap the descriptor label in a struct that also contains the
> > rcu_head struct required for deferring tasks using call_srcu() and stop
> > using kstrdup_const() as we're required to allocate memory anyway. Just
> > allocate enough for the label string and rcu_head in one go.
> >
> > Reported-by: Neil Armstrong <neil.armstrong@...aro.org>
> > Closes: https://lore.kernel.org/linux-gpio/CAMRc=Mfig2oooDQYTqo23W3PXSdzhVO4p=G4+P8y1ppBOrkrJQ@mail.gmail.com/
> > Fixes: 1f2bcb8c8ccd ("gpio: protect the descriptor label with SRCU")
> > Suggested-by: Paul E. McKenney <paulmck@...nel.org>
> > Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
>
> Looks good to me!
>
> Acked-by: Paul E. McKenney <paulmck@...nel.org>
>
> One semi-related question...  Why the per-descriptor srcu_struct?
>
> If the srcu_struct was shared among all of these, you could just do one
> synchronize_srcu() and one cleanup_srcu_struct() instead of needing to
> do one per gdev->desc[] entry.
>
> You might be able to go further and have one srcu_struct for all the
> gpio devices.
>
> Or did you guys run tests and find some performance problem with sharing
> srcu_struct structures?   (I wouldn't expect one, but sometimes the
> hardware has a better imagination than I do.)
>

I guess my goal was not to make synchronize_srcu() for descriptor X
wait for read-only operations on descriptor Y. But with that gone, I
suppose you're right, we can improve this patch further by switching
to a single SRCU descriptor.

I'll send a v2.

Bart