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From: Andy Lutomirski <luto@amacapital.net>
To: Rik van Riel <riel@surriel.com>
Cc: Andy Lutomirski <luto@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
	Mike Galbraith <efault@gmx.de>, kernel-team <kernel-team@fb.com>,
	Ingo Molnar <mingo@kernel.org>,
	Dave Hansen <dave.hansen@intel.com>
Subject: Re: [PATCH 4/7] x86,tlb: make lazy TLB mode lazier
Date: Wed, 18 Jul 2018 13:13:13 -1000	[thread overview]
Message-ID: <F93A3335-4830-4E36-9179-D0FA5D640C8A@amacapital.net> (raw)
In-Reply-To: <B6566AA4-2E8A-43BD-A224-0F6D4747B8FB@surriel.com>



> On Jul 18, 2018, at 10:58 AM, Rik van Riel <riel@surriel.com> wrote:
> 
> 
> 
>> On Jul 17, 2018, at 4:04 PM, Andy Lutomirski <luto@kernel.org> wrote:
>> 
>> 
>> I think you've introduced a minor-ish performance regression due to
>> changing the old (admittedly terribly documented) control flow a bit.
>> Before, if real_prev == next, we would skip:
>> 
>> load_mm_cr4(next);
>> switch_ldt(real_prev, next);
>> 
>> Now we don't any more.  I think you should reinstate that
>> optimization.  It's probably as simple as wrapping them in an if
>> (real_priv != next) with a comment like /* Remote changes that would
>> require a cr4 or ldt reload will unconditionally send an IPI even to
>> lazy CPUs.  So, if we aren't changing our mm, we don't need to refresh
>> cr4 or the ldt */
> 
> Looks like switch_ldt already skips reloading the LDT when prev equals
> next, or when they simply have the same LDT values:
> 
>        if (unlikely((unsigned long)prev->context.ldt |
>                     (unsigned long)next->context.ldt))
>                load_mm_ldt(next);
> 

Read that again?  It will reload if there’s an LDT, even if it’s the same one.

> It appears that the cr4 bits have a similar optimization:
> 
> static inline void cr4_set_bits(unsigned long mask)
> {
>        unsigned long cr4, flags;
> 
>        local_irq_save(flags);
>        cr4 = this_cpu_read(cpu_tlbstate.cr4);
>        if ((cr4 | mask) != cr4)
>                __cr4_set(cr4 | mask);
>        local_irq_restore(flags);
> }
>> 
>> Hmm.  load_mm_cr4() should bypass itself when mm == &init_mm.  Want to
>> fix that part or should I?
>> 
> Looks like there might not be anything to do here, after all.

But if init_mm and the thread that just went idle have different selected cr4 values, we’ll still write it.  With your lazy TLB work, it’s less of a big deal, but still.

I’m happy to fix this myself, though.

> 
> On to the lazy TLB mm_struct refcounting stuff :)
> 

Which refcount?  mm_users shouldn’t be hot, so I assume you’re talking about mm_count. My suggestion is to get rid of mm_count instead of trying to optimize it.

  reply	other threads:[~2018-07-18 23:13 UTC|newest]

Thread overview: 59+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-07-16 19:03 [PATCH v6 0/7] x86,tlb,mm: make lazy TLB mode even lazier Rik van Riel
2018-07-16 19:03 ` [PATCH 1/7] mm: allocate mm_cpumask dynamically based on nr_cpu_ids Rik van Riel
2018-07-17  9:33   ` [tip:x86/mm] mm: Allocate the mm_cpumask (mm->cpu_bitmap[]) " tip-bot for Rik van Riel
2018-08-04 22:28   ` [PATCH 1/7] mm: allocate mm_cpumask " Guenter Roeck
2018-07-16 19:03 ` [PATCH 2/7] x86,tlb: leave lazy TLB mode at page table free time Rik van Riel
2018-07-17  9:34   ` [tip:x86/mm] x86/mm/tlb: Leave " tip-bot for Rik van Riel
2018-07-17 11:46     ` Peter Zijlstra
2018-07-25  1:00       ` Anders Roxell
2018-08-16  1:54   ` [PATCH 2/7] x86,tlb: leave " Andy Lutomirski
2018-08-16  5:31     ` Rik van Riel
2018-07-16 19:03 ` [PATCH 3/7] x86,mm: restructure switch_mm_irqs_off Rik van Riel
2018-07-17  9:34   ` [tip:x86/mm] x86/mm/tlb: Restructure switch_mm_irqs_off() tip-bot for Rik van Riel
2018-10-09 14:58   ` tip-bot for Rik van Riel
2018-07-16 19:03 ` [PATCH 4/7] x86,tlb: make lazy TLB mode lazier Rik van Riel
2018-07-17  9:35   ` [tip:x86/mm] x86/mm/tlb: Make " tip-bot for Rik van Riel
2018-07-17 11:33     ` Peter Zijlstra
2018-07-18 15:33       ` Rik van Riel
2018-07-18 16:00         ` Peter Zijlstra
     [not found]           ` <081E558D-DB34-4A18-A35C-896BC47F6EBA@surriel.com>
2018-07-18 18:23             ` Peter Zijlstra
2018-07-18 18:51               ` Rik van Riel
2018-07-19  9:13                 ` Peter Zijlstra
2018-07-17 20:04   ` [PATCH 4/7] x86,tlb: make " Andy Lutomirski
     [not found]     ` <FF977B78-140F-4787-AA57-0EA934017D85@surriel.com>
2018-07-17 21:29       ` Andy Lutomirski
2018-07-17 22:05         ` Rik van Riel
2018-07-17 22:27           ` Andy Lutomirski
2018-07-18 20:58     ` Rik van Riel
2018-07-18 23:13       ` Andy Lutomirski [this message]
     [not found]         ` <B976CC13-D014-433A-83DE-F8DF9AB4F421@surriel.com>
2018-07-19 16:45           ` Andy Lutomirski
2018-07-19 17:04             ` Andy Lutomirski
2018-07-20  4:57               ` Benjamin Herrenschmidt
2018-07-20  8:30               ` Peter Zijlstra
2018-07-23 12:26                 ` Rik van Riel
2018-07-24 16:33               ` Will Deacon
     [not found]             ` <CF849A07-B7CE-4DE9-8246-53AC5A53A705@surriel.com>
2018-07-19 17:18               ` Andy Lutomirski
2018-07-20  8:02             ` Vitaly Kuznetsov
2018-07-20  9:49               ` Peter Zijlstra
2018-07-20 10:18                 ` Vitaly Kuznetsov
2018-07-20  9:32             ` Peter Zijlstra
2018-07-20 11:04               ` Peter Zijlstra
2018-07-16 19:03 ` [PATCH 5/7] x86,tlb: only send page table free TLB flush to lazy TLB CPUs Rik van Riel
2018-07-17  9:35   ` [tip:x86/mm] x86/mm/tlb: Only " tip-bot for Rik van Riel
2018-07-17 11:39     ` Peter Zijlstra
     [not found]       ` <1F8BDD25-864D-4105-B872-2109AA417454@surriel.com>
     [not found]         ` <24AA4367-22A1-450E-8F6A-3CBF39518384@surriel.com>
2018-07-18 16:19           ` Peter Zijlstra
2018-07-16 19:03 ` [PATCH 6/7] x86,mm: always use lazy TLB mode Rik van Riel
2018-07-17  9:36   ` [tip:x86/mm] x86/mm/tlb: Always " tip-bot for Rik van Riel
2018-10-09 14:58   ` tip-bot for Rik van Riel
2018-07-16 19:03 ` [PATCH 7/7] x86,switch_mm: skip atomic operations for init_mm Rik van Riel
2018-07-17  9:36   ` [tip:x86/mm] x86/mm/tlb: Skip atomic operations for 'init_mm' in switch_mm_irqs_off() tip-bot for Rik van Riel
  -- strict thread matches above, loose matches on Subject: below --
2018-07-10 14:28 [PATCH v5 0/7] x86,tlb,mm: make lazy TLB mode even lazier Rik van Riel
2018-07-10 14:28 ` [PATCH 4/7] x86,tlb: make lazy TLB mode lazier Rik van Riel
2018-07-06 21:56 [PATCH v4 0/7] x86,tlb,mm: make lazy TLB mode even lazier Rik van Riel
2018-07-06 21:56 ` [PATCH 4/7] x86,tlb: make lazy TLB mode lazier Rik van Riel
2018-06-29 14:29 [PATCH v3 0/7] x86,tlb,mm: make lazy TLB mode even lazier Rik van Riel
2018-06-29 14:29 ` [PATCH 4/7] x86,tlb: make lazy TLB mode lazier Rik van Riel
2018-06-29 17:05   ` Dave Hansen
2018-06-29 17:29     ` Rik van Riel
2018-06-20 19:56 [PATCH 0/7] x86,tlb,mm: make lazy TLB mode even lazier Rik van Riel
2018-06-20 19:56 ` [PATCH 4/7] x86,tlb: make lazy TLB mode lazier Rik van Riel
2018-06-22 15:04   ` Andy Lutomirski
2018-06-22 15:15     ` Rik van Riel
2018-06-22 15:34       ` Andy Lutomirski
2018-06-22 17:05   ` Dave Hansen
2018-06-22 17:16     ` Rik van Riel

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