linux-kernel - Re: [PATCH v2] filemap: add trace events for get_pages, map

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Date: Wed, 26 Jun 2024 09:58:12 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: Takaya Saeki <takayas@...omium.org>, Matthew Wilcox
 <willy@...radead.org>, Andrew Morton <akpm@...ux-foundation.org>, Mathieu
 Desnoyers <mathieu.desnoyers@...icios.com>, Junichi Uekawa
 <uekawa@...omium.org>, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [PATCH v2] filemap: add trace events for get_pages, map_pages,
 and fault

On Wed, 26 Jun 2024 21:31:57 +0900
Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:

> On Thu, 20 Jun 2024 16:19:03 +0000
> Takaya Saeki <takayas@...omium.org> wrote:
> 
> > To allow precise tracking of page caches accessed, add new tracepoints
> > that trigger when a process actually accesses them.
> > 
> > The ureadahead program used by ChromeOS traces the disk access of
> > programs as they start up at boot up. It uses mincore(2) or the
> > 'mm_filemap_add_to_page_cache' trace event to accomplish this. It stores
> > this information in a "pack" file and on subsequent boots, it will read
> > the pack file and call readahead(2) on the information so that disk
> > storage can be loaded into RAM before the applications actually need it.
> > 
> > A problem we see is that due to the kernel's readahead algorithm that
> > can aggressively pull in more data than needed (to try and accomplish
> > the same goal) and this data is also recorded. The end result is that
> > the pack file contains a lot of pages on disk that are never actually
> > used. Calling readahead(2) on these unused pages can slow down the
> > system boot up times.
> > 
> > To solve this, add 3 new trace events, get_pages, map_pages, and fault.
> > These will be used to trace the pages are not only pulled in from disk,
> > but are actually used by the application. Only those pages will be
> > stored in the pack file, and this helps out the performance of boot up.
> > 
> > With the combination of these 3 new trace events and
> > mm_filemap_add_to_page_cache, we observed a reduction in the pack file
> > by 7.3% - 20% on ChromeOS varying by device.
> >   
> 
> This looks good to me from the trace-event point of view.
> 
> Reviewed-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>

I added my reviewed-by on the last patch, you could have added it on
this one as it didn't change as much. But anyway, here it is again:

Reviewed-by: Steven Rostedt (Google) <rostedt@...dmis.org>

-- Steve