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 16:39:26 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Valentin Schneider <vschneid@redhat.com>
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, mtosatti@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: <20230406143926.GP386572@hirez.programming.kicks-ass.net>
References: <20230404134224.137038-1-ypodemsk@redhat.com>
 <20230404134224.137038-4-ypodemsk@redhat.com>
 <ZC1Q7uX4rNLg3vEg@lothringen>
 <ZC1XD/sEJY+zRujE@lothringen>
 <20230405114148.GA351571@hirez.programming.kicks-ass.net>
 <ZC1j8ivE/kK7+Gd5@lothringen>
 <xhsmhpm8ia46p.mognet@vschneid.remote.csb>
 <20230406133805.GO386572@hirez.programming.kicks-ass.net>
 <xhsmh8rf59k2f.mognet@vschneid.remote.csb>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <xhsmh8rf59k2f.mognet@vschneid.remote.csb>
Precedence: bulk

On Thu, Apr 06, 2023 at 03:11:52PM +0100, Valentin Schneider wrote:
> On 06/04/23 15:38, Peter Zijlstra wrote:
> > On Wed, Apr 05, 2023 at 01:45:02PM +0100, Valentin Schneider wrote:
> >>
> >> I've been hacking on something like this (CSD deferral for NOHZ-full),
> >> and unfortunately this uses the CPU-local cfd_data storage thing, which
> >> means any further smp_call_function() from the same CPU to the same
> >> destination will spin on csd_lock_wait(), waiting for the target CPU to
> >> come out of userspace and flush the queue - and we've just spent extra
> >> effort into *not* disturbing it, so that'll take a while :(
> >
> > I'm not sure I buy into deferring stuff.. a NOHZ_FULL cpu might 'never'
> > come back. Queueing data just in case it does seems wasteful.
> 
> Putting those callbacks straight into the bin would make my life much
> easier!

Well, it's either they get inhibited at the source like the parent patch
does, or they go through. I really don't see a sane middle way here.

> Unfortunately, even if they really should, I don't believe all of the
> things being crammed onto NOHZ_FULL CPUs have the same definition of
> 'never' as we do :/

That's not entirely the point, the point is that there are proper
NOHZ_FULL users that won't return to the kernel until the machine shuts
down. Buffering stuff for them is more or less a direct memory leak.