From a54b1873c0e3ffec3041cf72cb10d986d6bc4fe5 Mon Sep 17 00:00:00 2001 From: Chris Wilson Date: Fri, 24 Nov 2017 13:00:30 +0000 Subject: [PATCH] drm/i915: Use exponential backoff for wait_for() MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit Instead of sleeping for a fixed 1ms (roughly, depending on timer slack), start with a small sleep and exponentially increase the sleep on each cycle. A good example of a beneficiary is the guc mmio communication channel. Typically we expect (and so spin) for 10us for a quick response, but this doesn't cover everything and so sometimes we fallback to the millisecond+ sleep. This incurs a significant delay in time-critical operations like preemption (igt/gem_exec_latency), which can be improved significantly by using a small sleep after the spin fails. We've made this suggestion many times, but had little experimental data to support adding the complexity. v2: Bump the minimum usleep to 10us on advice of Documentation/timers/timers-howto.txt (Tvrko) v3: Specify min, max range for usleep intervals -- some code may crucially depend upon and so want to specify the sleep pattern. References: 1758b90e38f5 ("drm/i915: Use a hybrid scheme for fast register waits") Signed-off-by: Chris Wilson Cc: Tvrtko Ursulin Cc: John Harrison Cc: Michał Winiarski Cc: Ville Syrjala Reviewed-by: Michał Winiarski Link: https://patchwork.freedesktop.org/patch/msgid/20171124130031.20761-2-chris@chris-wilson.co.uk --- drivers/gpu/drm/i915/intel_drv.h | 11 +++++++---- drivers/gpu/drm/i915/intel_pm.c | 2 +- 2 files changed, 8 insertions(+), 5 deletions(-) diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h index 635a96f..c00441a 100644 --- a/drivers/gpu/drm/i915/intel_drv.h +++ b/drivers/gpu/drm/i915/intel_drv.h @@ -48,8 +48,9 @@ * having timed out, since the timeout could be due to preemption or similar and * we've never had a chance to check the condition before the timeout. */ -#define _wait_for(COND, US, W) ({ \ +#define _wait_for(COND, US, Wmin, Wmax) ({ \ unsigned long timeout__ = jiffies + usecs_to_jiffies(US) + 1; \ + long wait__ = (Wmin); /* recommended min for usleep is 10 us */ \ int ret__; \ might_sleep(); \ for (;;) { \ @@ -62,12 +63,14 @@ ret__ = -ETIMEDOUT; \ break; \ } \ - usleep_range((W), (W) * 2); \ + usleep_range(wait__, wait__ * 2); \ + if (wait__ < (Wmax)) \ + wait__ <<= 1; \ } \ ret__; \ }) -#define wait_for(COND, MS) _wait_for((COND), (MS) * 1000, 1000) +#define wait_for(COND, MS) _wait_for((COND), (MS) * 1000, 10, 1000) /* If CONFIG_PREEMPT_COUNT is disabled, in_atomic() always reports false. */ #if defined(CONFIG_DRM_I915_DEBUG) && defined(CONFIG_PREEMPT_COUNT) @@ -116,7 +119,7 @@ int ret__; \ BUILD_BUG_ON(!__builtin_constant_p(US)); \ if ((US) > 10) \ - ret__ = _wait_for((COND), (US), 10); \ + ret__ = _wait_for((COND), (US), 10, 10); \ else \ ret__ = _wait_for_atomic((COND), (US), 0); \ ret__; \ diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c index 03d67d8..a80c322 100644 --- a/drivers/gpu/drm/i915/intel_pm.c +++ b/drivers/gpu/drm/i915/intel_pm.c @@ -9294,7 +9294,7 @@ int skl_pcode_request(struct drm_i915_private *dev_priv, u32 mbox, u32 request, ret = 0; goto out; } - ret = _wait_for(COND, timeout_base_ms * 1000, 10); + ret = _wait_for(COND, timeout_base_ms * 1000, 10, 10); if (!ret) goto out; -- 2.7.4