linux-kernel - Re: [PATCH v2 2/2] mm/mmu_gather: send tlb_remove_table_smp

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Date:   Thu, 22 Jun 2023 16:14:58 +0300
From:   ypodemsk@...hat.com
To:     Dave Hansen <dave.hansen@...el.com>, mtosatti@...hat.com,
        ppandit@...hat.com, david@...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, hpa@...or.com,
        keescook@...omium.org, paulmck@...nel.org, frederic@...nel.org,
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        samitolvanen@...gle.com, juerg.haefliger@...onical.com,
        arnd@...db.de, rmk+kernel@...linux.org.uk, geert+renesas@...der.be,
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        aneesh.kumar@...ux.ibm.com, x86@...nel.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, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] mm/mmu_gather: send tlb_remove_table_smp_sync
 IPI only to MM CPUs

On Wed, 2023-06-21 at 10:42 -0700, Dave Hansen wrote:
> On 6/20/23 07:46, Yair Podemsky wrote:
> > -void tlb_remove_table_sync_one(void)
> > +#ifdef CONFIG_ARCH_HAS_CPUMASK_BITS
> > +#define REMOVE_TABLE_IPI_MASK mm_cpumask(mm)
> > +#else
> > +#define REMOVE_TABLE_IPI_MASK cpu_online_mask
> > +#endif /* CONFIG_ARCH_HAS_CPUMASK_BITS */
> > +
> > +void tlb_remove_table_sync_one(struct mm_struct *mm)
> >  {
> >  	/*
> >  	 * This isn't an RCU grace period and hence the page-tables
> > cannot be
> > @@ -200,7 +206,8 @@ void tlb_remove_table_sync_one(void)
> >  	 * It is however sufficient for software page-table walkers
> > that rely on
> >  	 * IRQ disabling.
> >  	 */
> > -	smp_call_function(tlb_remove_table_smp_sync, NULL, 1);
> > +	on_each_cpu_mask(REMOVE_TABLE_IPI_MASK,
> > tlb_remove_table_smp_sync,
> > +			NULL, true);
> >  }
> 
> That "REMOVE_TABLE_IPI_MASK" thing is pretty confusing.  It *looks*
> like
> a constant.  It does *NOT* look at all like it consumes 'mm'.  Worst
> case, just create a local variable:
> 
> 	if (IS_ENABLED(CONFIG_ARCH_HAS_CPUMASK_BITS))
> 		ipi_mask = mm_cpumask(mm);
> 	else
> 		ipi_mask = cpu_online_mask;
> 
> 	on_each_cpu_mask(ipi_mask, ...);
> 
> That's a billion times more clear and it'll compile down to the same
> thing.
> 
> I do think the CONFIG_ARCH_HAS_CPUMASK_BITS naming is also pretty
> confusing, but I don't have any better suggestions.  Maybe something
> with "MM_CPUMASK" in it?
> 

Hi Dave,
Thanks for your suggestions!
I will send a new version with the local variable as you suggested
soon.
As for the config name, what about CONFIG_ARCH_HAS_MM_CPUMASK?
Thanks,
Yair