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

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Date: Tue, 7 May 2024 20:38:21 +0300
From: Andy Shevchenko <andriy.shevchenko@...el.com>
To: Bartosz Golaszewski <brgl@...ev.pl>
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 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.

> Just allocate enough for the label string and rcu_head in one go.

-- 
With Best Regards,
Andy Shevchenko