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Message-ID: <ZC6/0hRXztNwqXg0@tpad>
Date: Thu, 6 Apr 2023 09:49:22 -0300
From: Marcelo Tosatti <mtosatti@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Frederic Weisbecker <frederic@...nel.org>,
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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 Wed, Apr 05, 2023 at 09:54:57PM +0200, Peter Zijlstra wrote:
> 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?
Yes, IPIs to vCPU (guest context). In this case we can consider
an IPI to the host pCPU (which requires VM-exit from guest context).
> 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.
I suppose its common practice to use certain system calls in latency
sensitive applications, for example nanosleep. Some examples:
1) cyclictest (nanosleep)
2) PLC programs (nanosleep)
A system call does not necessarily have to take locks, does it ?
Or even if application does system calls, but runs under a VM,
then you are requiring it to never VM-exit.
This reduces the flexibility of developing such applications.
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