linux-kernel - Re: [PATCH] x86/mm: Tracking linear mapping split events since boot

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Date:   Mon, 25 Jan 2021 12:15:51 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Saravanan D <saravanand@...look.com>, x86@...nel.org,
        dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH] x86/mm: Tracking linear mapping split events since boot

On 1/25/21 12:11 PM, Saravanan D wrote:
> Numerous hugepage splits in the linear mapping would give
> admins the signal to narrow down the sluggishness caused by TLB
> miss/reload.
> 
> One of the many lasting (as we don't coalesce back) sources for huge page
> splits is tracing as the granular page attribute/permission changes would
> force the kernel to split code segments mapped to huge pages to smaller
> ones thereby increasing the probability of TLB miss/reload even after
> tracing has been stopped.
> 
> The split event information will be displayed at the bottom of
> /proc/meminfo
> ....
> DirectMap4k:     3505112 kB
> DirectMap2M:    19464192 kB
> DirectMap1G:    12582912 kB
> DirectMap2MSplits:  1705
> DirectMap1GSplits:    20

This seems much more like something we'd want in /proc/vmstat or as a
tracepoint than meminfo.  A tracepoint would be especially nice because
the trace buffer could actually be examined if an admin finds an
excessive number of these.