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

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Message-ID: <alpine.LRH.2.02.1806121304530.1848@file01.intranet.prod.int.rdu2.redhat.com>
Date:   Tue, 12 Jun 2018 13:19:28 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Alan Stern <stern@...land.harvard.edu>
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, Alan Stern wrote:

> 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...

ksoftirqd has priority 0 - it is not suitable for real-time tasks, such as 
audio.

In my opinion, it is much easier to fix this in the ehci driver (by not 
offloading isochronous completions), than to design a new 
real-time-capable ksoftirqd.

> > > 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

snd_complete_urb is doing nothing but submitting the same urb again. Is 
resubmitting the urb really causing so much latency that you can't do it 
in the interrupt handler?

Mikulas