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

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Message-ID: <20230406133206.GN386572@hirez.programming.kicks-ass.net>
Date:   Thu, 6 Apr 2023 15:32:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        Yair Podemsky <ypodemsk@...hat.com>, linux@...linux.org.uk,
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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 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.

> 2) PLC programs		(nanosleep)

What's a PLC? Programmable Logic Circuit?

> A system call does not necessarily have to take locks, does it ?

This all is unrelated to locks

> 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.

> This reduces the flexibility of developing such applications.

Yeah, that's the cards you're dealt, deal with it.