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Date: Thu, 14 Nov 2013 23:08:11 -0500 From: Steven Rostedt <rostedt@...dmis.org> To: LKML <linux-kernel@...r.kernel.org> Cc: "Theodore Ts'o" <tytso@....edu>, Andreas Dilger <adilger.kernel@...ger.ca>, linux-ext4@...r.kernel.org Subject: [PATCH] ext4: Use trace condition for ext4_mballoc tracepoints Doing an if statement to test some condition to know if we should trigger a tracepoint is pointless when tracing is disabled. This just adds overhead and wastes a branch prediction. This is why the TRACE_EVENT_CONDITION() was created. It places the check inside the jump label so that the branch does not happen unless tracing is enabled. That is, instead of doing: if (em) trace_btrfs_get_extent(root, em); Which is basically this: if (em) if (static_key(trace_btrfs_get_extent)) { Using a TRACE_EVENT_CONDITION() we can just do: trace_btrfs_get_extent(root, em); And the condition trace event will do: if (static_key(trace_btrfs_get_extent)) { if (em) { ... The static key is a non conditional jump (or nop) that is faster than having to check if em is NULL or not. Signed-off-by: Steven Rostedt <rostedt@...dmis.org> -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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