linux-kernel - Re: [PATCH V2] x86/mm: Tracking linear mapping split events

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Message-ID: <5562757e-578b-0ad7-1a9c-c64238283ab7@intel.com>
Date:   Wed, 27 Jan 2021 13:32:03 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Tejun Heo <tj@...nel.org>, Saravanan D <saravanand@...com>
Cc:     x86@...nel.org, dave.hansen@...ux.intel.com, luto@...nel.org,
        peterz@...radead.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH V2] x86/mm: Tracking linear mapping split events

On 1/27/21 1:03 PM, Tejun Heo wrote:
>> The lifetime split event information will be displayed at the bottom of
>> /proc/vmstat
>> ....
>> swap_ra 0
>> swap_ra_hit 0
>> direct_map_2M_splits 139
>> direct_map_4M_splits 0
>> direct_map_1G_splits 7
>> nr_unstable 0
>> ....
> 
> This looks great to me.

Yeah, this looks fine to me.  It's way better than meminfo.

>>  arch/x86/mm/pat/set_memory.c  | 117 ++++++++++++++++++++++++++++++++++
>>  include/linux/vm_event_item.h |   8 +++
>>  mm/vmstat.c                   |   8 +++
>>  3 files changed, 133 insertions(+)
> 
> So, now the majority of the added code is to add debugfs knobs which don't
> provide anything that userland can't already do by simply reading the
> monotonic counters.
> 
> Dave, are you still set on the resettable counters?

Not *really*.  But, you either need them to be resettable, or you need
to expect your users to take snapshots and compare changes over time.
Considering how much more code it is, though, I'm not super attached to it.