linux-kernel - Re: [PATCH 2/3] perf/x86/intel/pt: Add an option to not force PSB+ on every schedule-in

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Message-ID: <20150730104310.GW19282@twins.programming.kicks-ass.net>
Date:	Thu, 30 Jul 2015 12:43:10 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	adrian.hunter@...el.com, x86@...nel.org, hpa@...or.com,
	acme@...radead.org
Subject: Re: [PATCH 2/3] perf/x86/intel/pt: Add an option to not force PSB+
 on every schedule-in

On Fri, Jul 17, 2015 at 04:34:09PM +0300, Alexander Shishkin wrote:
> Currently, the PT driver zeroes out the status register every time before
> starting the event. However, all the writable bits are already taken care
> of in pt_handle_status() function, except the new PacketByteCnt field,
> which in new versions of PT contains the number of packet bytes written
> since the last sync (PSB) packet. Zeroing it out before enabling PT forces
> a sync packet to be written. This means that, with the existing code, a
> sync packet (PSB and PSBEND, 18 bytes in total) will be generated every
> time a PT event is scheduled in.
> 
> To avoid these unnecessary syncs and save a WRMSR in the fast path, this
> patch adds a new attribute config bit "no_force_psb", which will disable
> this zeroing WRMSR.

Why is this exposed?

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