On 21/03/2017 11:33, Chris Wilson wrote:
Rather than impose the cost of a locked test before queuing a new
request, reduce it to a simple test_bit() with a following clear_bit()
prior to doing the CSB check. This ensure that if an interrupt does
occur whilst reading from the CSB, we still detect it (the interrupt
would trigger a rescheduling of the tasklet anyway).
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
---
drivers/gpu/drm/i915/intel_lrc.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 296d125d8665..3154b98dc971 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -530,13 +530,18 @@ static void intel_lrc_irq_handler(unsigned long data)
intel_uncore_forcewake_get(dev_priv, engine->fw_domains);
- while (test_and_clear_bit(ENGINE_IRQ_EXECLIST, &engine->irq_posted)) {
+ /* Prefer doing test_and_clear_bit() as a two stage operation to avoid
+ * imposing the cost of a locked atomic transaction when submitting a
+ * new request (outside of the context-switch interrupt).
+ */
+ while (test_bit(ENGINE_IRQ_EXECLIST, &engine->irq_posted)) {
u32 __iomem *csb_mmio =
dev_priv->regs + i915_mmio_reg_offset(RING_CONTEXT_STATUS_PTR(engine));
u32 __iomem *buf =
dev_priv->regs + i915_mmio_reg_offset(RING_CONTEXT_STATUS_BUF_LO(engine, 0));
unsigned int csb, head, tail;
+ clear_bit(ENGINE_IRQ_EXECLIST, &engine->irq_posted);
csb = readl(csb_mmio);
head = GEN8_CSB_READ_PTR(csb);
tail = GEN8_CSB_WRITE_PTR(csb);
Looks safe to me from the point of view of potential races. If a new
interrupt arrives and sets the bit just before the tasklet clears it, we
would process the full set of CSB updates on the following line.
I can also confirm that it has a real effect of bringing the CPU usage
of this interrupt handler down.
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Regards,
Tvrtko
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