Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Thu, 6 Apr 2023 09:49:22 -0300
From:   Marcelo Tosatti <mtosatti@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Frederic Weisbecker <frederic@kernel.org>,
        Yair Podemsky <ypodemsk@redhat.com>, linux@armlinux.org.uk,
        mpe@ellerman.id.au, npiggin@gmail.com, christophe.leroy@csgroup.eu,
        hca@linux.ibm.com, gor@linux.ibm.com, agordeev@linux.ibm.com,
        borntraeger@linux.ibm.com, svens@linux.ibm.com,
        davem@davemloft.net, tglx@linutronix.de, mingo@redhat.com,
        bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org,
        hpa@zytor.com, will@kernel.org, aneesh.kumar@linux.ibm.com,
        akpm@linux-foundation.org, arnd@arndb.de, keescook@chromium.org,
        paulmck@kernel.org, jpoimboe@kernel.org, samitolvanen@google.com,
        ardb@kernel.org, juerg.haefliger@canonical.com,
        rmk+kernel@armlinux.org.uk, geert+renesas@glider.be,
        tony@atomide.com, linus.walleij@linaro.org,
        sebastian.reichel@collabora.com, nick.hawkins@hpe.com,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org,
        sparclinux@vger.kernel.org, linux-arch@vger.kernel.org,
        linux-mm@kvack.org, vschneid@redhat.com, dhildenb@redhat.com,
        alougovs@redhat.com
Subject: Re: [PATCH 3/3] mm/mmu_gather: send tlb_remove_table_smp_sync IPI
 only to CPUs in kernel mode
Message-ID: <ZC6/0hRXztNwqXg0@tpad>
References: <20230404134224.137038-1-ypodemsk@redhat.com>
 <20230404134224.137038-4-ypodemsk@redhat.com>
 <ZC1Q7uX4rNLg3vEg@lothringen>
 <ZC3PUkI7N2uEKy6v@tpad>
 <20230405195457.GC365912@hirez.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20230405195457.GC365912@hirez.programming.kicks-ass.net>
Precedence: bulk

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.