[Intel-gfx] [PATCH] i915/query: Correlate engine and cpu timestamps with better accuracy

[Chris Wilson]

Quoting Umesh Nerlige Ramappa (2021-03-03 21:28:00)
> Perf measurements rely on CPU and engine timestamps to correlate
> events of interest across these time domains. Current mechanisms get
> these timestamps separately and the calculated delta between these
> timestamps lack enough accuracy.
> 
> To improve the accuracy of these time measurements to within a few us,
> add a query that returns the engine and cpu timestamps captured as
> close to each other as possible.
> 
> v2: (Tvrtko)
> - document clock reference used
> - return cpu timestamp always
> - capture cpu time just before lower dword of cs timestamp
> 
> v3: (Chris)
> - use uncore-rpm
> - use __query_cs_timestamp helper
> 
> v4: (Lionel)
> - Kernel perf subsytem allows users to specify the clock id to be used
>   in perf_event_open. This clock id is used by the perf subsystem to
>   return the appropriate cpu timestamp in perf events. Similarly, let
>   the user pass the clockid to this query so that cpu timestamp
>   corresponds to the clock id requested.
> 
> v5: (Tvrtko)
> - Use normal ktime accessors instead of fast versions
> - Add more uApi documentation
> 
> v6: (Lionel)
> - Move switch out of spinlock
> 
> v7: (Chris)
> - cs_timestamp is a misnomer, use cs_cycles instead
> - return the cs cycle frequency as well in the query
> 
> v8:
> - Add platform and engine specific checks
> 
> v9: (Lionel)
> - Return 2 cpu timestamps in the query - captured before and after the
>   register read
> 
> Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa at intel.com>
> ---
>  drivers/gpu/drm/i915/i915_query.c | 144 ++++++++++++++++++++++++++++++
>  include/uapi/drm/i915_drm.h       |  47 ++++++++++
>  2 files changed, 191 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_query.c b/drivers/gpu/drm/i915/i915_query.c
> index fed337ad7b68..acca22ee6014 100644
> --- a/drivers/gpu/drm/i915/i915_query.c
> +++ b/drivers/gpu/drm/i915/i915_query.c
> @@ -6,6 +6,8 @@
>  
>  #include <linux/nospec.h>
>  
> +#include "gt/intel_engine_pm.h"
> +#include "gt/intel_engine_user.h"
>  #include "i915_drv.h"
>  #include "i915_perf.h"
>  #include "i915_query.h"
> @@ -90,6 +92,147 @@ static int query_topology_info(struct drm_i915_private *dev_priv,
>         return total_length;
>  }
>  
> +typedef u64 (*__ktime_func_t)(void);
> +static __ktime_func_t __clock_id_to_func(clockid_t clk_id)
> +{
> +       /*
> +        * Use logic same as the perf subsystem to allow user to select the
> +        * reference clock id to be used for timestamps.
> +        */
> +       switch (clk_id) {
> +       case CLOCK_MONOTONIC:
> +               return &ktime_get_ns;
> +       case CLOCK_MONOTONIC_RAW:
> +               return &ktime_get_raw_ns;
> +       case CLOCK_REALTIME:
> +               return &ktime_get_real_ns;
> +       case CLOCK_BOOTTIME:
> +               return &ktime_get_boottime_ns;
> +       case CLOCK_TAI:
> +               return &ktime_get_clocktai_ns;
> +       default:
> +               return NULL;
> +       }
> +}
> +
> +static inline int
> +__read_timestamps(struct intel_uncore *uncore,
> +                 i915_reg_t lower_reg,
> +                 i915_reg_t upper_reg,
> +                 u64 *cs_ts,
> +                 u64 *cpu_ts,
> +                 __ktime_func_t cpu_clock)
> +{
> +       u32 upper, lower, old_upper, loop = 0;
> +
> +       upper = intel_uncore_read_fw(uncore, upper_reg);
> +       do {
> +               cpu_ts[0] = cpu_clock();
> +               lower = intel_uncore_read_fw(uncore, lower_reg);
> +               cpu_ts[1] = cpu_clock();
> +               old_upper = upper;
> +               upper = intel_uncore_read_fw(uncore, upper_reg);

Both register reads comprise the timestamp returned to userspace, so
presumably you want cpu_ts[] to wrap both.

       do {
               old_upper = upper;

               cpu_ts[0] = cpu_clock();
               lower = intel_uncore_read_fw(uncore, lower_reg);
               upper = intel_uncore_read_fw(uncore, upper_reg);
               cpu_ts[1] = cpu_clock();
       } while (upper != old_upper && loop++ < 2);
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