linux-kernel - Re: [PATCH V5] x86/mm: Tracking linear mapping split events

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Message-ID: <20210128212048.oopcyfdf4j2lc663@devvm1945.atn0.facebook.com>
Date:   Thu, 28 Jan 2021 13:20:48 -0800
From:   Saravanan D <saravanand@...com>
To:     Dave Hansen <dave.hansen@...el.com>
CC:     <x86@...nel.org>, <dave.hansen@...ux.intel.com>, <luto@...nel.org>,
        <peterz@...radead.org>, <corbet@....net>, <willy@...radead.org>,
        <linux-kernel@...r.kernel.org>, <kernel-team@...com>,
        <linux-doc@...r.kernel.org>, <linux-mm@...ck.org>,
        <songliubraving@...com>
Subject: Re: [PATCH V5] x86/mm: Tracking linear mapping split events

Hi Dave,
> 
> Eek.  There really doesn't appear to be a place in Documentation/ that
> we've documented vmstat entries.
> 
> Maybe you can start:
> 
> 	Documentation/admin-guide/mm/vmstat.rst
> 
I was also very surprised that there does not exist documentation for
vmstat, that lead me to add a page in admin-guide which now requires lot
of caveats.

Starting a new documentation for vmstat goes beyond the scope of this patch.
I am inclined to remove Documentation from the next version [V6] of the patch.

I presume that a detailed commit log [V6] explaining why direct mapped kernel
page splis will never coalesce, how kernel tracing causes some of those
splits and why it is worth tracking them can do the job.

Proposed [V6] Commit Log:
>>>
To help with debugging the sluggishness caused by TLB miss/reload,
we introduce monotonic hugepage [direct mapped] split event counts since
system state: SYSTEM_RUNNING to be displayed as part of
/proc/vmstat in x86 servers

The lifetime split event information will be displayed at the bottom of
/proc/vmstat
....
swap_ra 0
swap_ra_hit 0
direct_map_level2_splits 94
direct_map_level3_splits 4
nr_unstable 0
....

One of the many lasting sources of direct hugepage splits is kernel
tracing (kprobes, tracepoints).

Note that the kernel's code segment [512 MB] points to the same 
physical addresses that have been already mapped in the kernel's 
direct mapping range.

Source : Documentation/x86/x86_64/mm.rst

When we enable kernel tracing, the kernel has to modify attributes/permissions
of the text segment hugepages that are direct mapped causing them to split.

Kernel's direct mapped hugepages do not coalesce back after split and
remain in place for the remainder of the lifetime.

An instance of direct page splits when we turn on
dynamic kernel tracing
....
cat /proc/vmstat | grep -i direct_map_level
direct_map_level2_splits 784
direct_map_level3_splits 12
bpftrace -e 'tracepoint:raw_syscalls:sys_enter { @ [pid, comm] =
count(); }'
cat /proc/vmstat | grep -i
direct_map_level
direct_map_level2_splits 789
direct_map_level3_splits 12
....
<<<

Thanks,
Saravanan D