linux-kernel - [PATCH v2 0/7] x86/mm/tlb: make lazy TLB mode even lazier

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Message-Id: <20180926035844.1420-1-riel@surriel.com>
Date:   Tue, 25 Sep 2018 23:58:37 -0400
From:   Rik van Riel <riel@...riel.com>
To:     linux-kernel@...r.kernel.org
Cc:     peterz@...radead.org, kernel-team@...com, will.deacon@....com,
        songliubraving@...com, mingo@...nel.org, luto@...nel.org,
        hpa@...or.com, npiggin@...il.com
Subject: [PATCH v2 0/7] x86/mm/tlb: make lazy TLB mode even lazier

Linus asked me to come up with a smaller patch set to get the benefits
of lazy TLB mode, so I spent some time trying out various permutations
of the code, with a few workloads that do lots of context switches, and
also happen to have a fair number of TLB flushes a second.

Both of the workloads tested are memcache style workloads, running
on two socket systems. One of the workloads has around 300,000
context switches a second, and around 19,000 TLB flushes.

The first patch in the series, of always using lazy TLB mode,
reduces CPU use around 1% on both Haswell and Broadwell systems.

The rest of the series reduces the number of TLB flush IPIs by
about 1,500 a second, resulting in a 0.2% reduction in CPU use,
on top of the 1% seen by just enabling lazy TLB mode.

These are the low hanging fruits in the context switch code.

The big thing remaining is the reference count overhead of
the lazy TLB mm_struct, but getting rid of that is rather a
lot of code for a small performance gain. Not quite what
Linus asked for :)

This v2 is "identical" to the version I posted yesterday,
except this one is actually against current -tip (not sure
what went wrong before), with a number of relevant patches
on top:
- tip x86/core
	012e77a903d ("x86/nmi: Fix NMI uaccess race against CR3 switching")
- arm64 tlb/asm-generic (entire branch)
- peterz queue mm/tlb
	12b2b80ec6f4 ("x86/mm: Page size aware flush_tlb_mm_range()")