linux-kernel - Re: [PATCH 2/7] mm/page_alloc: Treat RT tasks similar to __GFP

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Date:   Fri, 13 Jan 2023 11:09:52 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     Michal Hocko <mhocko@...e.com>, Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        NeilBrown <neilb@...e.de>,
        Thierry Reding <thierry.reding@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/7] mm/page_alloc: Treat RT tasks similar to __GFP_HIGH

On Fri, Jan 13, 2023 at 09:04:50AM +0000, David Laight wrote:
> From: Mel Gorman
> > Sent: 12 January 2023 09:36
> ...
> > Hard realtime tasks should be locking down resources in advance. Even a
> > soft-realtime task like audio or video live decoding which cannot jitter
> > should be allocating both memory and any disk space required up-front
> > before the recording starts instead of relying on reserves. At best,
> > reserve access will only delay the problem by a very short interval.
> 
> Or, at least, ensuring the system isn't memory limited.
> 

Added to changelog.

> The biggest effect on RT task latency/jitter (on a normal kernel)
> is hardware interrupt and softint code 'stealing' the cpu.
> The main 'culprit' being ethernet receive.
>  
> Unfortunately if you are doing RTP audio (UDP data) you absolutely
> need the ethernet receive to run. When the softint code decides
> to drop back to a normal priority kernel worker thread packets
> get lost.
> 

Yes, although this is a fundamental problem for background networking
processing in general IIUC that is independent of mm/. ksoftirqd may be
getting stalled behind a higher priority, a realtime task, a task that has
built a credit due to sleep time under GENTLE_FAIR_SLEEPERS relative to
ksoftirqd etc. As a normal low-priority CPU hog may be at the same priority
as ksoftirqd, it can use enough of the scheduler slice for the runqueue to
cause an indirect priority inversion if a high priority task is sleeping
waiting on network traffic it needs ASAP that's stalled on ksoftirqd. I
didn't check for other examples but the only one I'm aware of that boosts
ksoftirq priority is during rcutorture (see rcutorture_booster_init). A
quick glance doesn't show any possibility of boosting ksoftirqd priority
temporarily if dealing with NET_TX_SOFTIRQ although it might be an
interesting idea if it was demonstrated with a realistic (or at least
semi-realistic) test case that priority inversion can occur due to background
RX processing. It's not even PREEMPT_RT-specific, I suspect it's a general
problem. I don't think this series makes a difference because if any of
the critical tasks are depending on the memory reserves, they're already
in serious trouble.

> (I've been running 10000 RTP streams - with 10k receive UDP sockets.)
> 

min_reserve access isn't going to fix any stalls with that volume of
streams :D

> So I doubt avoiding sleeps in kmalloc() is going to make a
> significant difference.
> 

Agreed.

-- 
Mel Gorman
SUSE Labs