linux-kernel - Re: [PATCH] ipmi_si_intf: use usleep

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Message-ID: <20190709220908.GL657710@devbig004.ftw2.facebook.com>
Date:   Tue, 9 Jul 2019 15:09:08 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Corey Minyard <minyard@....org>
Cc:     openipmi-developer@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH] ipmi_si_intf: use usleep_range() instead of busy looping

Hello, Corey.

On Tue, Jul 09, 2019 at 04:46:02PM -0500, Corey Minyard wrote:
> I'm also a little confused because the CPU in question shouldn't
> be doing anything else if the schedule() immediately returns here,
> so it's not wasting CPU that could be used on another process.  Or
> is it lock contention that is causing an issue on other CPUs?

Yeah, pretty pronounced too and it also keeps the CPU busy which makes
the load balancer deprioritize that CPU.  Busy looping is never free.

> IMHO, this whole thing is stupid; if you design hardware with
> stupid interfaces (byte at a time, no interrupts) you should
> expect to get bad performance.  But I can't control what the
> hardware vendors do.  This whole thing is a carefully tuned
> compromise.

I'm really not sure "carefully tuned" is applicable on indefinite busy
looping.

> So I can't really take this as-is.

We can go for shorter timeouts for sure but I don't think this sort of
busy looping is acceptable.  Is your position that this must be a busy
loop?

Thanks.

-- 
tejun