linux-kernel - Re: [RFC][PATCH 1/2] sched: Extended scheduler time slice

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Message-ID: <20250210215116.4cbf240a@pumpkin>
Date: Mon, 10 Feb 2025 21:51:16 +0000
From: David Laight <david.laight.linux@...il.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Joel Fernandes <joel@...lfernandes.org>, Prakash Sangappa
 <prakash.sangappa@...cle.com>, Peter Zijlstra <peterz@...radead.org>,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, Thomas
 Gleixner <tglx@...utronix.de>, Ankur Arora <ankur.a.arora@...cle.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>, linux-mm@...ck.org,
 x86@...nel.org, Andrew Morton <akpm@...ux-foundation.org>, luto@...nel.org,
 bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
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 raghavendra.kt@....com, Boris Ostrovsky <boris.ostrovsky@...cle.com>,
 Konrad Wilk <konrad.wilk@...cle.com>, jgross@...e.com,
 Andrew.Cooper3@...rix.com, Vineeth Pillai <vineethrp@...gle.com>, Suleiman
 Souhlal <suleiman@...gle.com>, Ingo Molnar <mingo@...nel.org>, Mathieu
 Desnoyers <mathieu.desnoyers@...icios.com>, Clark Williams
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 Joseph Salisbury <joseph.salisbury@...cle.com>, broonie@...il.com
Subject: Re: [RFC][PATCH 1/2] sched: Extended scheduler time slice

On Mon, 10 Feb 2025 14:44:32 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Mon, 10 Feb 2025 12:27:00 -0500
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > On Mon, 10 Feb 2025 17:20:59 +0000
> > David Laight <david.laight.linux@...il.com> wrote:
> >   
> > > Hmmm... you lose big-time anyway.
> > > 
> > > All you need is a lot of network traffic 'pinch' the process context until
> > > the hardware interrupt, NAPI softint code and rcu softint code completes.
> > > That can easily take several milliseconds.    
> > 
> > Not on PREEMPT_RT. All that runs as threads.
> > 
> > And this is a feature that we would like on RT for non RT tasks.  
> 
> Actually, this doesn't even need PREEMPT_RT to not hit your example. You
> can build and boot your system with interrupts as threads, and that also
> includes softirqs.

And then you lose lots of receive ethernet packets unless you change
all the thread priorities.
(I don't recall anything that makes them run at a low FIFO prioriry.)

And don't mention the mess that happens if you have hardware that is
raising an interrupt ever 10ms and really needs the ISR to run within 10ms.
Someone tried to do that into a VM, the interrupts turn up like busses.
None come for ages and then three arrive together :-)

	David