Re: [PATCH] mm/vmstat: Add events for PMD based THP migration without split

From: John Hubbard
Date: Mon Jun 01 2020 - 23:20:12 EST

On 2020-06-01 09:57, Daniel Jordan wrote:
Hi Anshuman,

On Fri, May 22, 2020 at 09:04:04AM +0530, Anshuman Khandual wrote:
This adds the following two new VM events which will help in validating PMD
based THP migration without split. Statistics reported through these events
will help in performance debugging.


The names suggest a binary event similar to the existing
pgmigrate_success/fail, but FAILURE only tracks one kind of migration error,
and then only when the thp is successfully split, so shouldn't it be called
SPLIT instead?

So the full description of the situation, which we're trying to compress into
a shorter name, is "THP pmd migration failure, due to successfully splitting
the THP". From that, the beginning part is the real point here, while the last
part is less important. In other words, the users of these events are people
who are trying to quantify THP migrations, and these events are particularly
relevant for that. The "THP migration failed" is more important here than
the reason that it failed. Or so I believe so far.

So I still think that the names are really quite good, but your point is
also important: maybe this patch should also be tracking other causes
of THP PMD migration failure, in order to get a truer accounting of the

John Hubbard