linux-kernel - Re: [PATCH v2] x86: enable RCU based table free

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Date:   Thu, 24 Aug 2017 17:27:21 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        xen-devel@...ts.xenproject.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jork Loeser <Jork.Loeser@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH v2] x86: enable RCU based table free

Peter Zijlstra <peterz@...radead.org> writes:

> On Thu, Aug 24, 2017 at 11:22:58AM +0200, Vitaly Kuznetsov wrote:
>
>> diff --git a/arch/x86/include/asm/tlb.h b/arch/x86/include/asm/tlb.h
>> index c7797307fc2b..d43a7fcafee9 100644
>> --- a/arch/x86/include/asm/tlb.h
>> +++ b/arch/x86/include/asm/tlb.h
>> @@ -15,4 +15,9 @@
>>  
>>  #include <asm-generic/tlb.h>
>>  
>> +static inline void __tlb_remove_table(void *table)
>> +{
>> +	free_page_and_swap_cache(table);
>> +}
>
> Most other archs have this in pgtable.h, only ARM* has it in tlb.h.
>

Sure, I can move it in v3 if nobody objects.

> And should we put a comment on explaining _why_ we have RCU_TABLE_FREE
> enabled?

Do you think adding something like

/*
 * While x86 architecture in general requires an IPI to perform TLB
 * shootdown, enablement code for several hypervisors overrides
 * .flush_tlb_others hook in pv_mmu_ops and implements it by issuing
 * a hypercall. To keep software pagetable walkers safe in this case we 
 * switch to RCU based table free (HAVE_RCU_TABLE_FREE). See the comment
 * below 'ifdef CONFIG_HAVE_RCU_TABLE_FREE' in include/asm-generic/tlb.h
 * for more details.
 */

before __tlb_remove_table would suffice? Or do you see a better place
for such comment?

Actually, after enabling HAVE_RCU_TABLE_FREE on x86 we may consider
switching to this mechanism globally: it seems to have negligible effect
on performace (and all major arches will already have it). One step at a
time, though.

-- 
  Vitaly