From 741258cdd297e94b5f7167408b958b55795abaf8 Mon Sep 17 00:00:00 2001 From: Tvrtko Ursulin Date: Thu, 26 Apr 2018 08:47:16 +0100 Subject: [PATCH] drm/i915: Use seqlock in engine stats We can convert engine stats from a spinlock to seqlock to ensure interrupt processing is never even a tiny bit delayed by parallel readers. There is a smidgen bit more cost on the write lock side, and an extremely unlikely chance that readers will have to retry a few times in face of heavy interrupt load. But it should be extremely unlikely given how lightweight read side section is compared to the interrupt processing side, and also compared to the rest of the code paths which can lead into it. Furthermore, writer is the ones doing the real, latency sensitive work, while readers are only informative. Signed-off-by: Tvrtko Ursulin Suggested-by: Chris Wilson Cc: Chris Wilson Reviewed-by: Chris Wilson Link: https://patchwork.freedesktop.org/patch/msgid/20180426074716.7352-1-tvrtko.ursulin@linux.intel.com --- drivers/gpu/drm/i915/intel_engine_cs.c | 19 ++++++++++--------- drivers/gpu/drm/i915/intel_ringbuffer.h | 11 ++++++----- 2 files changed, 16 insertions(+), 14 deletions(-) diff --git a/drivers/gpu/drm/i915/intel_engine_cs.c b/drivers/gpu/drm/i915/intel_engine_cs.c index 58be7fac5b8c..ac009f10c948 100644 --- a/drivers/gpu/drm/i915/intel_engine_cs.c +++ b/drivers/gpu/drm/i915/intel_engine_cs.c @@ -306,7 +306,7 @@ intel_engine_setup(struct drm_i915_private *dev_priv, /* Nothing to do here, execute in order of dependencies */ engine->schedule = NULL; - spin_lock_init(&engine->stats.lock); + seqlock_init(&engine->stats.lock); ATOMIC_INIT_NOTIFIER_HEAD(&engine->context_status_notifier); @@ -1481,7 +1481,7 @@ int intel_enable_engine_stats(struct intel_engine_cs *engine) return -ENODEV; tasklet_disable(&execlists->tasklet); - spin_lock_irqsave(&engine->stats.lock, flags); + write_seqlock_irqsave(&engine->stats.lock, flags); if (unlikely(engine->stats.enabled == ~0)) { err = -EBUSY; @@ -1505,7 +1505,7 @@ int intel_enable_engine_stats(struct intel_engine_cs *engine) } unlock: - spin_unlock_irqrestore(&engine->stats.lock, flags); + write_sequnlock_irqrestore(&engine->stats.lock, flags); tasklet_enable(&execlists->tasklet); return err; @@ -1534,12 +1534,13 @@ static ktime_t __intel_engine_get_busy_time(struct intel_engine_cs *engine) */ ktime_t intel_engine_get_busy_time(struct intel_engine_cs *engine) { + unsigned int seq; ktime_t total; - unsigned long flags; - spin_lock_irqsave(&engine->stats.lock, flags); - total = __intel_engine_get_busy_time(engine); - spin_unlock_irqrestore(&engine->stats.lock, flags); + do { + seq = read_seqbegin(&engine->stats.lock); + total = __intel_engine_get_busy_time(engine); + } while (read_seqretry(&engine->stats.lock, seq)); return total; } @@ -1557,13 +1558,13 @@ void intel_disable_engine_stats(struct intel_engine_cs *engine) if (!intel_engine_supports_stats(engine)) return; - spin_lock_irqsave(&engine->stats.lock, flags); + write_seqlock_irqsave(&engine->stats.lock, flags); WARN_ON_ONCE(engine->stats.enabled == 0); if (--engine->stats.enabled == 0) { engine->stats.total = __intel_engine_get_busy_time(engine); engine->stats.active = 0; } - spin_unlock_irqrestore(&engine->stats.lock, flags); + write_sequnlock_irqrestore(&engine->stats.lock, flags); } #if IS_ENABLED(CONFIG_DRM_I915_SELFTEST) diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h index c5e27905b0e1..24af3f1088ba 100644 --- a/drivers/gpu/drm/i915/intel_ringbuffer.h +++ b/drivers/gpu/drm/i915/intel_ringbuffer.h @@ -3,6 +3,7 @@ #define _INTEL_RINGBUFFER_H_ #include +#include #include "i915_gem_batch_pool.h" #include "i915_gem_timeline.h" @@ -595,7 +596,7 @@ struct intel_engine_cs { /** * @lock: Lock protecting the below fields. */ - spinlock_t lock; + seqlock_t lock; /** * @enabled: Reference count indicating number of listeners. */ @@ -1064,7 +1065,7 @@ static inline void intel_engine_context_in(struct intel_engine_cs *engine) if (READ_ONCE(engine->stats.enabled) == 0) return; - spin_lock_irqsave(&engine->stats.lock, flags); + write_seqlock_irqsave(&engine->stats.lock, flags); if (engine->stats.enabled > 0) { if (engine->stats.active++ == 0) @@ -1072,7 +1073,7 @@ static inline void intel_engine_context_in(struct intel_engine_cs *engine) GEM_BUG_ON(engine->stats.active == 0); } - spin_unlock_irqrestore(&engine->stats.lock, flags); + write_sequnlock_irqrestore(&engine->stats.lock, flags); } static inline void intel_engine_context_out(struct intel_engine_cs *engine) @@ -1082,7 +1083,7 @@ static inline void intel_engine_context_out(struct intel_engine_cs *engine) if (READ_ONCE(engine->stats.enabled) == 0) return; - spin_lock_irqsave(&engine->stats.lock, flags); + write_seqlock_irqsave(&engine->stats.lock, flags); if (engine->stats.enabled > 0) { ktime_t last; @@ -1109,7 +1110,7 @@ static inline void intel_engine_context_out(struct intel_engine_cs *engine) } } - spin_unlock_irqrestore(&engine->stats.lock, flags); + write_sequnlock_irqrestore(&engine->stats.lock, flags); } int intel_enable_engine_stats(struct intel_engine_cs *engine); -- 2.45.2