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drm/i915: Use exponential backoff for wait_for()

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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: 1758b90e38 ("drm/i915: Use a hybrid scheme for fast register waits")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: John Harrison <John.C.Harrison@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171124130031.20761-2-chris@chris-wilson.co.uk
This commit is contained in:
Chris Wilson 2017-11-24 13:00:30 +00:00
parent 8c5da3bbcf
commit a54b1873c0
2 changed files with 8 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

11
drivers/gpu/drm/i915/intel_drv.h
View File

@ -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__; \

2
drivers/gpu/drm/i915/intel_pm.c
View File

@ -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;