linux-kernel - Re: [PATCH RESEND v3 5/6] AHCI: Optimize single IRQ interrupt processing

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Message-ID: <20141006072710.GH8971@agordeev.usersys.redhat.com>
Date:	Mon, 6 Oct 2014 08:27:11 +0100
From:	Alexander Gordeev <agordeev@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	linux-kernel@...r.kernel.org, linux-ide@...r.kernel.org
Subject: Re: [PATCH RESEND v3 5/6] AHCI: Optimize single IRQ interrupt
 processing

On Sun, Oct 05, 2014 at 12:16:46PM -0400, Tejun Heo wrote:
> I think the thing which bothers me is that due to softirq we end up
> bouncing the context twice.  IRQ schedules threaded IRQ handler after
> doing minimal amount of work.  The threaded IRQ handler gets scheduled
> and again it doesn't do much but basically just schedules block
> softirq to actually run completions which is the heavier part.
> Apparently this doesn't seem to hurt measureably but it's just weird.

Hi Tejun,

That is exactly the point I was concerned with when stated in one of
changelogs "The downside of this change is introduction of a kernel
thread". Splitting the service routine in two parts is a small change
(in terms of code familiarity). Yet it right away provides benefits I
could observe and justify (to myself at least).

> Why are we bouncing the context twice?

I *did* consider moving the threaded handler code to the softirq part.
I just wanted to get updates in stages: to address hardware interrupts
latency first and possibly threaded hander next. Getting done these two
together would be too big change for me ;)

> -- 
> tejun

-- 
Regards,
Alexander Gordeev
agordeev@...hat.com
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