netdev - Re: [PATCH net-next 1/4] i40e: avoid spin loop in i40e_asq_send

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Message-ID: <YXqOL0PqhujmH+sd@cork>
Date:   Thu, 28 Oct 2021 04:49:03 -0700
From:   Jörn Engel <joern@...estorage.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Tony Nguyen <anthony.l.nguyen@...el.com>, davem@...emloft.net,
        Caleb Sander <csander@...estorage.com>, netdev@...r.kernel.org,
        sassmann@...hat.com, Tony Brelinski <tony.brelinski@...el.com>
Subject: Re: [PATCH net-next 1/4] i40e: avoid spin loop in
 i40e_asq_send_command()

On Wed, Oct 27, 2021 at 09:01:03AM -0700, Jakub Kicinski wrote:
> On Mon, 25 Oct 2021 10:55:05 -0700 Tony Nguyen wrote:
> > +			cond_resched();
> >  			udelay(50);
> 
> Why not switch to usleep_range() if we can sleep here?

Looking at usleep_range() vs. udelay(), I wonder if there is still a
hidden reason to prefer udelay().  Basically, if you typically want
short delays like the 50µs above, going to sleep will often result in
much longer delays, 1ms or higher.  I can easily see situations where
multiple calls to udelay(50) are fine while multiple calls to
usleep_range() will cause timeouts.

Is that a known problem and do we have good heuristics when to prefer
one over the other?

Jörn

--
Audacity augments courage; hesitation, fear.
-- Publilius Syrus