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

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Message-ID: <1362573785.31874.36.camel@gandalf.local.home>
Date:	Wed, 06 Mar 2013 07:43:05 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Hiraku Toyooka <hiraku.toyooka.gu@...achi.com>
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?

On Wed, 2013-03-06 at 20:54 +0900, Hiraku Toyooka wrote:
> 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.

And I have many more to come :-)

> 
> > 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.)

Yeah, I may have been the one to bring it up, but I was wrong. After
playing with it, it doesn't make sense.

> 
> 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?

I already have a patch. I'll send it out later today, and perhaps you
can give me your "Acked-by".

Thanks,

-- Steve


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