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.
next prev parent 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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