linux-kernel - Re: [PATCH 3/3] mm/mmu_gather: send tlb_remove_table_smp

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Date:   Wed, 19 Apr 2023 08:01:41 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        Yair Podemsky <ypodemsk@...hat.com>, linux@...linux.org.uk,
        mpe@...erman.id.au, npiggin@...il.com, christophe.leroy@...roup.eu,
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        alougovs@...hat.com
Subject: Re: [PATCH 3/3] mm/mmu_gather: send tlb_remove_table_smp_sync IPI
 only to CPUs in kernel mode

On Thu, Apr 06, 2023 at 03:32:06PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 06, 2023 at 09:49:22AM -0300, Marcelo Tosatti wrote:
> 
> > > > 2) Depends on the application and the definition of "occasional".
> > > > 
> > > > For certain types of applications (for example PLC software or
> > > > RAN processing), upon occurrence of an event, it is necessary to
> > > > complete a certain task in a maximum amount of time (deadline).
> > > 
> > > If the application is properly NOHZ_FULL and never does a kernel entry,
> > > it will never get that IPI. If it is a pile of shit and does kernel
> > > entries while it pretends to be NOHZ_FULL it gets to keep the pieces and
> > > no amount of crying will get me to care.
> > 
> > I suppose its common practice to use certain system calls in latency
> > sensitive applications, for example nanosleep. Some examples:
> > 
> > 1) cyclictest		(nanosleep)
> 
> cyclictest is not a NOHZ_FULL application, if you tihnk it is, you're
> deluded.

On the field (what end-users do on production):

cyclictest runs on NOHZ_FULL cores.
PLC type programs run on NOHZ_FULL cores.

So accordingly to physical reality i observe, i am not deluded.

> > 2) PLC programs		(nanosleep)
> 
> What's a PLC? Programmable Logic Circuit?

Programmable logic controller.

> > A system call does not necessarily have to take locks, does it ?
> 
> This all is unrelated to locks

OK.

> > Or even if application does system calls, but runs under a VM,
> > then you are requiring it to never VM-exit.
> 
> That seems to be a goal for performance anyway.

Not sure what you mean.

> > This reduces the flexibility of developing such applications.
> 
> Yeah, that's the cards you're dealt, deal with it.

This is not what happens on the field.