Re: [RFC 5/7] drm/i915: Wait for GPU to finish before event stop in Gen Perf PMU

From: Chris Wilson <chris@chris-wilson.co.uk>
To: Robert Bragg <robert@sixbynine.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>,
	"intel-gfx@lists.freedesktop.org"
	<intel-gfx@lists.freedesktop.org>,
	"Wu, Jabin" <jabin.wu@intel.com>,
	"Gupta, Sourab" <sourab.gupta@intel.com>,
	"Woo, Insoo" <insoo.woo@intel.com>,
	"Bragg, Robert" <robert.bragg@intel.com>
Subject: Re: [RFC 5/7] drm/i915: Wait for GPU to finish before event stop in Gen Perf PMU
Date: Thu, 25 Jun 2015 18:37:18 +0100	[thread overview]
Message-ID: <20150625173718.GR30757@nuc-i3427.alporthouse.com> (raw)
In-Reply-To: <CAMou1-3XT6hgDX=oX4-sZWEupm0EoQOwAxQjGttPiyee_V1g_A@mail.gmail.com>

On Thu, Jun 25, 2015 at 06:31:52PM +0100, Robert Bragg wrote:
> Thinking about the details of waiting for the last RPC command before
> destroying the dest buffer and disabling OACONTROL these are the
> requirements I see:
> - we want free the dest buffer in a finite time, since it's large
> (i.e. don't want to assume it's ok to keep around if allocated once)

Skipping to this point, no you don't. If you mark the buffer as
purgeable after you stop tracking it (though they may still be pending
writes from the GPU), the system will preferentially reuse the buffer's
backing storage when memory is tight (including waiting for the GPU).
Similarly, you need to unpin it as soon as possible then the address
space is available for reuse asap.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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