linux-kernel - [PATCH 0/2 v2] tracing: Simplify and document the trace event filtering temp buffer code

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Message-ID: <20210609224309.945509725@goodmis.org>
Date:   Wed, 09 Jun 2021 18:43:09 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     linux-kernel@...r.kernel.org
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH 0/2 v2] tracing: Simplify and document the trace event filtering temp buffer code

When filtering trace events, a temp buffer is used because the extra copy
from the temp buffer into the ring buffer is still faster than the direct
write into the ring buffer followed by a discard if the filter does not
match.

But the data that can be stored in the temp buffer is a PAGE_SIZE minus the
ring buffer event header. The calculation of that header size is complex,
but using the helper macro "struct_size()" can simplify it.

Also, add more documentation about what is going on.

Link: https://lore.kernel.org/stable/CAHk-=whKbJkuVmzb0hD3N6q7veprUrSpiBHRxVY=AffWZPtxmg@mail.gmail.com/

Changes since v1:
  Switch "unlikely" to "likely"
  Test max_len <= instead of just <

Steven Rostedt (VMware) (2):
      tracing: Simplify the max length test when using the filtering temp buffer
      tracing: Add better comments for the filtering temp buffer use case

----
 kernel/trace/trace.c | 40 ++++++++++++++++++++++++++++++++++++++--
 1 file changed, 38 insertions(+), 2 deletions(-)