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Message-ID: <20230406133805.GO386572@hirez.programming.kicks-ass.net>
Date:   Thu, 6 Apr 2023 15:38:05 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Valentin Schneider <vschneid@...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,
        hca@...ux.ibm.com, gor@...ux.ibm.com, agordeev@...ux.ibm.com,
        borntraeger@...ux.ibm.com, svens@...ux.ibm.com,
        davem@...emloft.net, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, will@...nel.org, aneesh.kumar@...ux.ibm.com,
        akpm@...ux-foundation.org, arnd@...db.de, keescook@...omium.org,
        paulmck@...nel.org, jpoimboe@...nel.org, samitolvanen@...gle.com,
        ardb@...nel.org, juerg.haefliger@...onical.com,
        rmk+kernel@...linux.org.uk, geert+renesas@...der.be,
        tony@...mide.com, linus.walleij@...aro.org,
        sebastian.reichel@...labora.com, nick.hawkins@....com,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-mm@...ck.org, mtosatti@...hat.com, dhildenb@...hat.com,
        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 01:45:02PM +0100, Valentin Schneider wrote:
> On 05/04/23 14:05, Frederic Weisbecker wrote:
> >  static void smp_call_function_many_cond(const struct cpumask *mask,
> >                                       smp_call_func_t func, void *info,
> > @@ -946,10 +948,13 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
> >  #endif
> >                       cfd_seq_store(pcpu->seq_queue, this_cpu, cpu, CFD_SEQ_QUEUE);
> >                       if (llist_add(&csd->node.llist, &per_cpu(call_single_queue, cpu))) {
> > -				__cpumask_set_cpu(cpu, cfd->cpumask_ipi);
> > -				nr_cpus++;
> > -				last_cpu = cpu;
> > -
> > +				if (!(scf_flags & SCF_NO_USER) ||
> > +				    !IS_ENABLED(CONFIG_GENERIC_ENTRY) ||
> > +				     ct_state_cpu(cpu) != CONTEXT_USER) {
> > +					__cpumask_set_cpu(cpu, cfd->cpumask_ipi);
> > +					nr_cpus++;
> > +					last_cpu = cpu;
> > +				}
> 
> 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.

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