linux-kernel - Re: [PATCH v2 8/9] x86/mm/tlb: Remove UV special case

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Date:   Tue, 9 Jul 2019 21:09:13 +0000
From:   Nadav Amit <namit@...are.com>
To:     Mike Travis <mike.travis@....com>
CC:     Russ Anderson <rja@....com>, Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Dimitri Sivanich <sivanich@....com>,
        Hedi Berriche <hedi.berriche@....com>
Subject: Re: [PATCH v2 8/9] x86/mm/tlb: Remove UV special case

> On Jul 9, 2019, at 1:29 PM, Mike Travis <mike.travis@....com> wrote:
> 
> 
> 
> On 7/9/2019 1:09 PM, Russ Anderson wrote:
>> On Tue, Jul 09, 2019 at 09:50:27PM +0200, Thomas Gleixner wrote:
>>> On Tue, 2 Jul 2019, Nadav Amit wrote:
>>> 
>>>> SGI UV support is outdated and not maintained, and it is not clear how
>>>> it performs relatively to non-UV. Remove the code to simplify the code.
>>> 
>>> You should at least Cc the SGI/HP folks on that. They are still
>>> around. Done so.
>> Thanks Thomas.  The SGI UV is now HPE Superdome Flex and is
>> very much still supported.
>> Thanks.
>>>> Cc: Peter Zijlstra <peterz@...radead.org>
>>>> Cc: Dave Hansen <dave.hansen@...el.com>
>>>> Suggested-by: Andy Lutomirski <luto@...nel.org>
>>>> Signed-off-by: Nadav Amit <namit@...are.com>
>>>> ---
>>>>  arch/x86/mm/tlb.c | 25 -------------------------
>>>>  1 file changed, 25 deletions(-)
>>>> 
>>>> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
>>>> index b47a71820f35..64afe1215495 100644
>>>> --- a/arch/x86/mm/tlb.c
>>>> +++ b/arch/x86/mm/tlb.c
>>>> @@ -689,31 +689,6 @@ void native_flush_tlb_multi(const struct cpumask *cpumask,
>>>>  		trace_tlb_flush(TLB_REMOTE_SEND_IPI,
>>>>  				(info->end - info->start) >> PAGE_SHIFT);
>>>>  -	if (is_uv_system()) {
>>>> -		/*
>>>> -		 * This whole special case is confused.  UV has a "Broadcast
>>>> -		 * Assist Unit", which seems to be a fancy way to send IPIs.
>>>> -		 * Back when x86 used an explicit TLB flush IPI, UV was
>>>> -		 * optimized to use its own mechanism.  These days, x86 uses
>>>> -		 * smp_call_function_many(), but UV still uses a manual IPI,
>>>> -		 * and that IPI's action is out of date -- it does a manual
>>>> -		 * flush instead of calling flush_tlb_func_remote().  This
>>>> -		 * means that the percpu tlb_gen variables won't be updated
>>>> -		 * and we'll do pointless flushes on future context switches.
>>>> -		 *
>>>> -		 * Rather than hooking native_flush_tlb_multi() here, I think
>>>> -		 * that UV should be updated so that smp_call_function_many(),
>>>> -		 * etc, are optimal on UV.
>>>> -		 */
> 
> I thought this change was already proposed a bit ago and we acked it
> awhile back. Also the replacement functionality is being worked on but it
> is more complex. The smp call many has to support all the reasons why it’s
> called and not just the tlb shoot downs as is the current BAU case.

Sorry for not cc’ing you before. In the meanwhile, can you give an explicit
acked-by? (I couldn’t find the previous patch you regarded.)

Thanks,
Nadav