linux-kernel - Re: snapshot error on non allocated buffer?

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Message-ID: <51372E5A.2070504@hitachi.com>
Date:	Wed, 06 Mar 2013 20:54:02 +0900
From:	Hiraku Toyooka <hiraku.toyooka.gu@...achi.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: snapshot error on non allocated buffer?

Hi Steven,

(03/06/2013 12:50 AM), Steven Rostedt wrote:
> Hi Hiraku,
>
> I'm doing a lot of reconstruction of ftrace's buffering, and I'm also
> modifying a lot of the snapshot feature to work with the new stuff
> that's coming.
>

Many thanks. I'm trying your multi-buffer patches.

> I'm looking at the -EINVAL when you write something other than '0' or
> '1' into the snapshot file when the snapshot is not allocated. I'm
> thinking that it should just return as if it succeeded. I don't
> understand why it should return -EINVAL?
>

I thought that it might be a little strange if the clear operation
succeeded in spite of the non-allocated buffer.
(Actually, I simply implemented as you said, though.)

But I don't have trouble even if it succeeds, so I'll modify the I/F
to make it return successfully.

> Now if you want to know if the snapshot is allocated or not, I have a
> patch that shows how to use the snapshot feature when the snapshot is
> empty, and also give the status of the snapshot itself:
>
> [root] # cat /debug/tracing/snapshot
> # tracer: nop
> #
> #
> # * Snapshot is freed *
> #
> # Snapshot commands:
> # echo 0 > snapshot : Clears and frees snapshot buffer
> # echo 1 > snapshot : Allocates snapshot buffer, if not already
> allocated.
> #                      Takes a snapshot of the main buffer.
> # echo 2 > snapshot : Clears snapshot buffer (but does not allocate)
> #                      (Doesn't have to be '2' works with any number
> that
> #                       is not a '0' or '1')
>
> [root] # echo 1 > /debug/tracing/snapshot
> [root] # echo 2 > /debug/tracing/snapshot
> [root] # cat /debug/tracing/snapshot
> # tracer: nop
> #
> #
> # * Snapshot is allocated *
> #
> # Snapshot commands:
> # echo 0 > snapshot : Clears and frees snapshot buffer
> # echo 1 > snapshot : Allocates snapshot buffer, if not already
> allocated.
> #                      Takes a snapshot of the main buffer.
> # echo 2 > snapshot : Clears snapshot buffer (but does not allocate)
> #                      (Doesn't have to be '2' works with any number
> that
> #                       is not a '0' or '1')
>

This seems good for me and also users.

>
> As this is a new feature for 3.9, and we are still in -rc1, I think this
> might be a good thing to add now. As well as not returning -EINVAL on
> writing to the file when the snapshot buffer isn't allocated.
>
> What do you think?
>

I think it's OK.
I'll send a patch to make the file not return -EINVAL. Does it need to
be based on 3.9-rc1 or tip tree?

Thanks,
Hiraku Toyooka

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