linux-kernel - RE: [for-next][PATCH 12/17] fgraph: Make overruns 4 bytes in graph stack structure

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Date:   Thu, 12 Nov 2020 09:18:21 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Steven Rostedt' <rostedt@...dmis.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: RE: [for-next][PATCH 12/17] fgraph: Make overruns 4 bytes in graph
 stack structure

From: Steven Rostedt
> Sent: 12 November 2020 00:33
> 
> Inspecting the data structures of the function graph tracer, I found that
> the overrun value is unsigned long, which is 8 bytes on a 64 bit machine,
> and not only that, the depth is an int (4 bytes). The overrun can be simply
> an unsigned int (4 bytes) and pack the ftrace_graph_ret structure better.
> 
> The depth is moved up next to the func, as it is used more often with func,
> and improves cache locality.
...
>  } __packed;

Does this many any/much difference given that the structure is
marked __packed?

OTOH the __packed will (probably) kill performance on systems
that don't support mis-aligned accesses.

	David

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