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

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Date: Thu, 9 May 2024 14:38:49 +0200
From: Bartosz Golaszewski <brgl@...ev.pl>
To: Andy Shevchenko <andriy.shevchenko@...el.com>
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>, 
	"Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH] gpiolib: fix the speed of descriptor label setting with SRCU

On Tue, May 7, 2024 at 7:38 PM Andy Shevchenko
<andriy.shevchenko@...el.com> 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.
>
> If there is no label and we assign something like "?" (two bytes) we got
> with your patch the allocation of most likely 32 bytes (as next power of
> two for the SLAB) instead of 18..24.
>
> OTOH, I dunno if SLAB supports 24-bytes. If not, it means that up to 16 bytes
> label there would be no difference. In any case, with a new update (as far
> as I understood the next move) it might return to kstrdup_const() or so.
>

Memory is cheap. This is just done for requested lines of which there
are never that many. I wouldn't stress about it. The rcu_head struct
is already 32 bytes on its own.

Bart