[Intel-gfx] [RFC PATCH 157/162] drm/i915: Improve accuracy of eviction stats

[Chris Wilson]

Quoting Matthew Auld (2020-11-27 12:07:13)
> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> 
> Current code uses jiffie time to do the accounting and then does:
> 
>   diff = jiffies - start;
>   msec = diff * 1000 / HZ;
>   ...
>   atomic_long_add(msec, &i915->time_swap_out_ms);
> 
> If we assume jiffie can be as non-granular as 10ms and that the current
> accounting records all evictions faster than one jiffie as infinite speed,
> we can end up over-estimating the reported eviction throughput.
> 
> Fix this by accumulating ktime_t and only dividing to more user friendly
> granularity at presentation time (debugfs read).
> 
> At the same time consolidate the code a bit and convert from multiple
> atomics to single seqlock per stat.
> 
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Cc: CQ Tang <cq.tang at intel.com>
> Cc: Sudeep Dutt <sudeep.dutt at intel.com>
> Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>

A lot of effort to fix up patches after the fact, might as well make it
a real PMU interface.
-Chris