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

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Message-ID: <20230405195457.GC365912@hirez.programming.kicks-ass.net>
Date:   Wed, 5 Apr 2023 21:54:57 +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,
        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 Wed, Apr 05, 2023 at 04:43:14PM -0300, Marcelo Tosatti wrote:

> Two points:
> 
> 1) For a virtualized system, the overhead is not only of executing the
> IPI but:
> 
> 	VM-exit
> 	run VM-exit code in host
> 	handle IPI
> 	run VM-entry code in host
> 	VM-entry

I thought we could do IPIs without VMexit these days? Also virt... /me
walks away.

> 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.