linux-kernel - Re: [PATCH] usb: don't offload isochronous urb completions to ksoftirq

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Message-ID: <Pine.LNX.4.44L0.1806121232460.1582-100000@iolanthe.rowland.org>
Date:   Tue, 12 Jun 2018 12:38:48 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Mikulas Patocka <mpatocka@...hat.com>
cc:     Ming Lei <ming.lei@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        USB list <linux-usb@...r.kernel.org>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: don't offload isochronous urb completions to ksoftirq

On Tue, 12 Jun 2018, Mikulas Patocka wrote:

> > How about making the softirq thread's priority adjustable?
> 
> But you would have to argue with softirq maintainers about it - and you 
> say that you don't have time for that.

But maybe _you_ do...

> > As for coordinating with the softirq maintainers -- whether I want to 
> > or not isn't the issue.  Right now I don't have _time_ to do it.
> > 
> > Alan Stern
> 
> I am wondering - whats the purpose of that patch 
> 428aac8a81058e2303677a8fbf26670229e51d3a at all? The patch shows some 
> performance difference, but they are minor, about 1%.
> 
> If you want to call the urb callback as soon as possible - why don't you 
> just call it? Why do you need to offload the callback to a softirq thread?

Please read the Changelog entry for commit 94dfd7edfd5c.  Basically the 
idea was to reduce overall latency by not doing as much work in an 
interrupt handler.

Alan Stern