From: Peter Hurley <peter@hurleysoftware.com>
To: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>,
Francois Romieu <romieu@fr.zoreil.com>,
Eric Dumazet <edumazet@google.com>,
David Miller <davem@davemloft.net>,
netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
Greg KH <gregkh@linuxfoundation.org>,
dmaengine@vger.kernel.org,
John Ogness <john.ogness@linutronix.de>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
Andrew Morton <akpm@linux-foundation.org>,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: Softirq priority inversion from "softirq: reduce latencies"
Date: Mon, 29 Feb 2016 10:53:25 -0800 [thread overview]
Message-ID: <56D493A5.403@hurleysoftware.com> (raw)
In-Reply-To: <1456770279.648.85.camel@edumazet-ThinkPad-T530>
On 02/29/2016 10:24 AM, Eric Dumazet wrote:
> On lun., 2016-02-29 at 10:05 -0800, Peter Hurley wrote:
>
>> While I appreciate the attempt, that's not the problem.
>>
>> Just to be clear
>>
>> if (time_before(jiffies, end) && !need_resched() &&
>> --max_restart)
>> goto restart;
>>
>> aborts softirq *even if 0ns have elapsed*, if NET_RX has woken a process.
>
>
> Sure, now remove the 1st and 2nd condition.
Well just removing the 2nd condition has everything working fine,
because that fixes the priority inversion.
> You would still 'abort' (ie wakeup ksoftirqd really) when --max_restart
> becomes 0
Sure. Which would mean there's contended heavy i/o load so the driver
has to fallback to non-DMA. That's an acceptable outcome.
> So, instead of some subtle load dependent bug, you know have a reliable
> trigger.
There's no "subtle load dependent bug" here.
The driver has a fallback mode of operation that it relies on without
DMA. Of course, as I already wrote, this has consequences.
If system resources are _actually contended_, then naturally, fighting
for cpu and i/o time is fine, and I'm happy to do that in ksoftirqd.
However, when system resources are _not_ contended, it makes no
sense to be forced to revert to ksoftirqd resolution, which is strictly
intended as fallback.
Or flipping your argument on its head, why not just _always_ execute
softirq in ksoftirqd?
next prev parent reply other threads:[~2016-02-29 18:53 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-02-27 18:19 Softirq priority inversion from "softirq: reduce latencies" Peter Hurley
2016-02-27 20:13 ` Eric Dumazet
2016-02-27 20:29 ` Peter Hurley
2016-02-27 23:04 ` David Miller
2016-02-27 23:33 ` Peter Hurley
2016-02-28 1:59 ` Eric Dumazet
2016-02-28 2:10 ` Peter Hurley
2016-02-28 2:17 ` Eric Dumazet
2016-02-28 4:46 ` David Miller
2016-02-28 5:55 ` Mike Galbraith
2016-02-28 17:01 ` Francois Romieu
2016-02-29 4:58 ` Mike Galbraith
2016-02-29 15:03 ` Peter Hurley
2016-02-29 15:19 ` Eric Dumazet
2016-02-29 15:54 ` Peter Hurley
2016-02-29 16:21 ` Eric Dumazet
2016-02-29 18:05 ` Peter Hurley
2016-02-29 18:24 ` Eric Dumazet
2016-02-29 18:53 ` Peter Hurley [this message]
2016-02-29 19:14 ` Thomas Gleixner
2016-02-29 20:24 ` David Miller
2016-02-29 23:04 ` Peter Hurley
2016-02-29 15:27 ` Eric Dumazet
2016-02-29 19:13 ` Peter Hurley
2016-02-29 19:43 ` Eric Dumazet
2016-02-29 15:40 ` Mike Galbraith
2016-02-29 15:58 ` Peter Hurley
2016-02-29 16:24 ` Eric Dumazet
2016-02-29 17:16 ` David Miller
2016-03-07 15:31 ` Sebastian Andrzej Siewior
2016-03-07 17:06 ` Mike Galbraith
2016-03-07 15:48 ` Sebastian Andrzej Siewior
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