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Subject: Re: [PATCH] fs/proc: introduce /proc/stat2 file
To:     Davidlohr Bueso <dave@stgolabs.net>, akpm@linux-foundation.org
Cc:     linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Davidlohr Bueso <dbueso@suse.de>
References: <20181029192521.23059-1-dave@stgolabs.net>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <65a3a4b8-bb7b-5cb2-3f50-af4b2717f81d@redhat.com>
Date:   Mon, 29 Oct 2018 17:01:32 -0400
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On 10/29/2018 03:25 PM, Davidlohr Bueso wrote:
> A recent report from a large database vendor which I shall not name
> shows concerns about poor performance when consuming /proc/stat info.
> Particularly  kstat_irq() pops up in the profiles and most time is
> being spent there. The overall system is under a lot of irqs and
> almost 1k cores, thus this comes to little surprise.
>
> Granted that procfs in general is not known for its performance,
> nor designed for it, for that matter. Some users, however may be able
> to overcome this performance limitation, some not. Therefore it isn't
> bad having a kernel option for users that don't want any hard irq info
> -- and care enough about this.
>
> This patch introduces a new /proc/stat2 file that is identical to the
> regular 'stat' except that it zeroes all hard irq statistics. The new
> file is a drop in replacement to stat for users that need performance.
>
> The stat file is not touched, of course -- this was also previously
> suggested by Waiman:
> https://lore.kernel.org/lkml/1524166562-5644-1-git-send-email-longman@redhat.com/
>
> Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
> ---
>  Documentation/filesystems/proc.txt | 12 +++++++---
>  fs/proc/stat.c                     | 45 ++++++++++++++++++++++++++++++++------
>  2 files changed, 47 insertions(+), 10 deletions(-)
>
> diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
> index 12a5e6e693b6..563b01decb1e 100644
> --- a/Documentation/filesystems/proc.txt
> +++ b/Documentation/filesystems/proc.txt
> @@ -27,7 +27,7 @@ Table of Contents
>    1.5	SCSI info
>    1.6	Parallel port info in /proc/parport
>    1.7	TTY info in /proc/tty
> -  1.8	Miscellaneous kernel statistics in /proc/stat
> +  1.8	Miscellaneous kernel statistics in /proc/stat and /proc/stat2
>    1.9	Ext4 file system parameters
>  
>    2	Modifying System Parameters
> @@ -140,6 +140,7 @@ Table 1-1: Process specific entries in /proc
>   mem		Memory held by this process
>   root		Link to the root directory of this process
>   stat		Process status
> + stat2		Process status without irq information
>   statm		Process memory status information
>   status		Process status in human readable form
>   wchan		Present with CONFIG_KALLSYMS=y: it shows the kernel function
> @@ -1301,8 +1302,8 @@ To see  which  tty's  are  currently in use, you can simply look into the file
>    unknown              /dev/tty        4    1-63 console 
>  
>  
> -1.8 Miscellaneous kernel statistics in /proc/stat
> --------------------------------------------------
> +1.8 Miscellaneous kernel statistics in /proc/stat and /proc/stat2
> +-----------------------------------------------------------------
>  
>  Various pieces   of  information about  kernel activity  are  available in the
>  /proc/stat file.  All  of  the numbers reported  in  this file are  aggregates
> @@ -1371,6 +1372,11 @@ of the possible system softirqs. The first column is the total of all
>  softirqs serviced; each subsequent column is the total for that particular
>  softirq.
>  
> +The stat2 file acts as a performance alternative to /proc/stat for workloads

A "performant alternative", right?

-Longman