linux-kernel - Re: [RFC] mm/migration: Add trace events for THP migrations

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Date:   Mon, 3 Jan 2022 09:24:38 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-mm@...ck.org, Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Zi Yan <ziy@...dia.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        John Hubbard <jhubbard@...dia.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC] mm/migration: Add trace events for THP migrations

Hello Matthew,

On 12/24/21 8:18 PM, Matthew Wilcox wrote:
> On Fri, Dec 24, 2021 at 12:16:38PM +0530, Anshuman Khandual wrote:
>> This adds two trace events for PMD based THP migration without split. These
>> events closely follow the implementation details like setting and removing
>> of PMD migration entries, which are essential operations for THP migration.
> 
> Why are you printing the address of a struct page?  What useful
> information does this supply?  Same question for the struct mm.
> And the pmdp, for that matter.

Just to make individual trace records comprehensive enough to capture
which (and where) the PMD entries went through migration entry state.
But is there any particular concern here capturing mm, page and pmdp ?

> 
> You haven't said _why_ you want these tracepoints.  So it's impossible
> to suggest what you _should_ be doing, because what you _are_ doing
> is obviously wrong.

Just for debug purpose. To see which (and where) PMD entries are being
migrated as is without a split, via PMD migration entries. Wondering if
you are suggesting just to capture addr, pmdval and just drop others ?

> 
>> +	TP_printk("mm=%p, address=%lx, pmdp=%p, page=%p pmdval=%lx",
>> +		__entry->mm,
>> +		__entry->address,
>> +		__entry->pmdp,
>> +		__entry->page,
>> +		__entry->pmdval)
>> +);
> 

- Anshuman