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Date: Mon, 13 Jun 2016 13:42:37 +0900
From: Minchan Kim <minchan@kernel.org>
To: Ganesh Mahendran <opensource.ganesh@gmail.com>
CC: Linux-MM <linux-mm@kvack.org>, linux-kernel <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        <rostedt@goodmis.org>, <mingo@redhat.com>
Subject: Re: [PATCH] mm/zsmalloc: add trace events for zs_compact
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On Wed, Jun 08, 2016 at 02:39:19PM +0800, Ganesh Mahendran wrote:

<snip>

> zsmalloc is not only used by zram, but also zswap. Maybe
> others in the future.
> 
> I tried to use function_graph. It seems there are too much log
> printed:
> ------
> root@leo-test:/sys/kernel/debug/tracing# cat trace
> # tracer: function_graph
> #
> # CPU  DURATION                  FUNCTION CALLS
> # |     |   |                     |   |   |   |
>  2)               |  zs_compact [zsmalloc]() {
>  2)               |  /* zsmalloc_compact_start: pool zram0 */
>  2)   0.889 us    |    _raw_spin_lock();
>  2)   0.896 us    |    isolate_zspage [zsmalloc]();
>  2)   0.938 us    |    _raw_spin_lock();
>  2)   0.875 us    |    isolate_zspage [zsmalloc]();
>  2)   0.942 us    |    _raw_spin_lock();
>  2)   0.962 us    |    isolate_zspage [zsmalloc]();
> ...
>  2)   0.879 us    |      insert_zspage [zsmalloc]();
>  2)   4.520 us    |    }
>  2)   0.975 us    |    _raw_spin_lock();
>  2)   0.890 us    |    isolate_zspage [zsmalloc]();
>  2)   0.882 us    |    _raw_spin_lock();
>  2)   0.894 us    |    isolate_zspage [zsmalloc]();
>  2)               |  /* zsmalloc_compact_end: pool zram0: 0 pages
> compacted(total 0) */
>  2) # 1351.241 us |  }
> ------
> => 1351.241 us used
> 
> And it seems the overhead of function_graph is bigger than trace event.
> 
> bash-3682  [002] ....  1439.180646: zsmalloc_compact_start: pool zram0
> bash-3682  [002] ....  1439.180659: zsmalloc_compact_end: pool zram0:
> 0 pages compacted(total 0)
> => 13 us > 1351.241 us

You could use set_ftrace_filter to cut out.

To introduce new event trace to get a elasped time, it's pointless,
I think.

It should have more like pool name you mentioned.
Like saying other thread, It would be better to show
[pool name, compact size_class,
the number of object moved, the number of freed page], IMO.

Thanks.