[PATCH 1/1] i915/query: Correlate engine and cpu timestamps with better accuracy

Umesh Nerlige Ramappa umesh.nerlige.ramappa at intel.com
Fri Apr 30 22:26:09 UTC 2021


On Thu, Apr 29, 2021 at 02:07:58PM -0500, Jason Ekstrand wrote:
>On Wed, Apr 28, 2021 at 7:34 PM Umesh Nerlige Ramappa
><umesh.nerlige.ramappa at intel.com> wrote:
>>
>> 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
>>
>> v10: (Chris)
>> - Use local_clock() to measure time taken to read lower dword of
>>   register and return it to user.
>>
>> v11: (Jani)
>> - IS_GEN deprecated. User GRAPHICS_VER instead.
>>
>> v12: (Jason)
>> - Split cpu timestamp array into timestamp and delta for cleaner API
>>
>> Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa at intel.com>
>> Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
>> ---
>>  drivers/gpu/drm/i915/i915_query.c | 148 ++++++++++++++++++++++++++++++
>>  include/uapi/drm/i915_drm.h       |  52 +++++++++++
>>  2 files changed, 200 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/i915/i915_query.c b/drivers/gpu/drm/i915/i915_query.c
>> index fed337ad7b68..357c44e8177c 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 +92,151 @@ 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,
>> +                 u64 *cpu_delta,
>> +                 __ktime_func_t cpu_clock)
>> +{
>> +       u32 upper, lower, old_upper, loop = 0;
>> +
>> +       upper = intel_uncore_read_fw(uncore, upper_reg);
>> +       do {
>> +               *cpu_delta = local_clock();
>> +               *cpu_ts = cpu_clock();
>> +               lower = intel_uncore_read_fw(uncore, lower_reg);
>> +               *cpu_delta = local_clock() - *cpu_delta;
>> +               old_upper = upper;
>> +               upper = intel_uncore_read_fw(uncore, upper_reg);
>> +       } while (upper != old_upper && loop++ < 2);
>> +
>> +       *cs_ts = (u64)upper << 32 | lower;
>> +
>> +       return 0;
>> +}
>> +
>> +static int
>> +__query_cs_cycles(struct intel_engine_cs *engine,
>> +                 u64 *cs_ts, u64 *cpu_ts, u64 *cpu_delta,
>> +                 __ktime_func_t cpu_clock)
>> +{
>> +       struct intel_uncore *uncore = engine->uncore;
>> +       enum forcewake_domains fw_domains;
>> +       u32 base = engine->mmio_base;
>> +       intel_wakeref_t wakeref;
>> +       int ret;
>> +
>> +       fw_domains = intel_uncore_forcewake_for_reg(uncore,
>> +                                                   RING_TIMESTAMP(base),
>> +                                                   FW_REG_READ);
>> +
>> +       with_intel_runtime_pm(uncore->rpm, wakeref) {
>> +               spin_lock_irq(&uncore->lock);
>> +               intel_uncore_forcewake_get__locked(uncore, fw_domains);
>> +
>> +               ret = __read_timestamps(uncore,
>> +                                       RING_TIMESTAMP(base),
>> +                                       RING_TIMESTAMP_UDW(base),
>> +                                       cs_ts,
>> +                                       cpu_ts,
>> +                                       cpu_delta,
>> +                                       cpu_clock);
>> +
>> +               intel_uncore_forcewake_put__locked(uncore, fw_domains);
>> +               spin_unlock_irq(&uncore->lock);
>> +       }
>> +
>> +       return ret;
>> +}
>> +
>> +static int
>> +query_cs_cycles(struct drm_i915_private *i915,
>> +               struct drm_i915_query_item *query_item)
>> +{
>> +       struct drm_i915_query_cs_cycles __user *query_ptr;
>> +       struct drm_i915_query_cs_cycles query;
>> +       struct intel_engine_cs *engine;
>> +       __ktime_func_t cpu_clock;
>> +       int ret;
>> +
>> +       if (GRAPHICS_VER(i915) < 6)
>> +               return -ENODEV;
>> +
>> +       query_ptr = u64_to_user_ptr(query_item->data_ptr);
>> +       ret = copy_query_item(&query, sizeof(query), sizeof(query), query_item);
>> +       if (ret != 0)
>> +               return ret;
>> +
>> +       if (query.flags)
>> +               return -EINVAL;
>> +
>> +       if (query.rsvd)
>> +               return -EINVAL;
>> +
>> +       cpu_clock = __clock_id_to_func(query.clockid);
>> +       if (!cpu_clock)
>> +               return -EINVAL;
>> +
>> +       engine = intel_engine_lookup_user(i915,
>> +                                         query.engine.engine_class,
>> +                                         query.engine.engine_instance);
>> +       if (!engine)
>> +               return -EINVAL;
>> +
>> +       if (GRAPHICS_VER(i915) == 6 &&
>> +           query.engine.engine_class != I915_ENGINE_CLASS_RENDER)
>> +               return -ENODEV;
>> +
>> +       query.cs_frequency = engine->gt->clock_frequency;
>> +       ret = __query_cs_cycles(engine,
>> +                               &query.cs_cycles,
>> +                               &query.cpu_timestamp,
>> +                               &query.cpu_delta,
>> +                               cpu_clock);
>> +       if (ret)
>> +               return ret;
>> +
>> +       if (put_user(query.cs_frequency, &query_ptr->cs_frequency))
>> +               return -EFAULT;
>> +
>> +       if (put_user(query.cpu_timestamp, &query_ptr->cpu_timestamp))
>> +               return -EFAULT;
>> +
>> +       if (put_user(query.cpu_delta, &query_ptr->cpu_delta))
>> +               return -EFAULT;
>> +
>> +       if (put_user(query.cs_cycles, &query_ptr->cs_cycles))
>> +               return -EFAULT;
>> +
>> +       return sizeof(query);
>> +}
>> +
>>  static int
>>  query_engine_info(struct drm_i915_private *i915,
>>                   struct drm_i915_query_item *query_item)
>> @@ -424,6 +571,7 @@ static int (* const i915_query_funcs[])(struct drm_i915_private *dev_priv,
>>         query_topology_info,
>>         query_engine_info,
>>         query_perf_config,
>> +       query_cs_cycles,
>>  };
>>
>>  int i915_query_ioctl(struct drm_device *dev, void *data, struct drm_file *file)
>> diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
>> index 6a34243a7646..0b4c27092d41 100644
>> --- a/include/uapi/drm/i915_drm.h
>> +++ b/include/uapi/drm/i915_drm.h
>> @@ -2230,6 +2230,10 @@ struct drm_i915_query_item {
>>  #define DRM_I915_QUERY_TOPOLOGY_INFO    1
>>  #define DRM_I915_QUERY_ENGINE_INFO     2
>>  #define DRM_I915_QUERY_PERF_CONFIG      3
>> +       /**
>> +        * Query Command Streamer timestamp register.
>> +        */
>> +#define DRM_I915_QUERY_CS_CYCLES       4
>>  /* Must be kept compact -- no holes and well documented */
>>
>>         /**
>> @@ -2397,6 +2401,54 @@ struct drm_i915_engine_info {
>>         __u64 rsvd1[4];
>>  };
>>
>> +/**
>> + * struct drm_i915_query_cs_cycles
>> + *
>> + * The query returns the command streamer cycles and the frequency that can be
>> + * used to calculate the command streamer timestamp. In addition the query
>> + * returns a set of cpu timestamps that indicate when the command streamer cycle
>> + * count was captured.
>> + */
>> +struct drm_i915_query_cs_cycles {
>> +       /** Engine for which command streamer cycles is queried. */
>> +       struct i915_engine_class_instance engine;
>
>I've checked with HW engineers and they're claiming that all CS
>timestamp registers should report the same time modulo minor drift.
>You're CC'd on the internal e-mail.  If this is really the case, then
>I don't think we want to put an engine in this query.

Looks like the engine can be dropped since all timestamps are in sync. I 
just have one more question here. The timestamp itself is 36 bits.  
Should the uapi also report the timestamp width to the user OR should I 
just return the lower 32 bits of the timestamp?

Thanks,
Umesh

>
>--Jason
>
>> +
>> +       /** Must be zero. */
>> +       __u32 flags;
>> +
>> +       /**
>> +        * Command streamer cycles as read from the command streamer
>> +        * register at 0x358 offset.
>> +        */
>> +       __u64 cs_cycles;
>> +
>> +       /** Frequency of the cs cycles in Hz. */
>> +       __u64 cs_frequency;
>> +
>> +       /**
>> +        * CPU timestamp in ns. The timestamp is captured before reading the
>> +        * cs_cycles register using the reference clockid set by the user.
>> +        */
>> +       __u64 cpu_timestamp;
>> +
>> +       /**
>> +        * Time delta in ns captured around reading the lower dword of the
>> +        * cs_cycles register.
>> +        */
>> +       __u64 cpu_delta;
>> +
>> +       /**
>> +        * Reference clock id for CPU timestamp. For definition, see
>> +        * clock_gettime(2) and perf_event_open(2). Supported clock ids are
>> +        * CLOCK_MONOTONIC, CLOCK_MONOTONIC_RAW, CLOCK_REALTIME, CLOCK_BOOTTIME,
>> +        * CLOCK_TAI.
>> +        */
>> +       __s32 clockid;
>> +
>> +       /** Must be zero. */
>> +       __u32 rsvd;
>> +};
>> +
>>  /**
>>   * struct drm_i915_query_engine_info
>>   *
>> --
>> 2.20.1
>>


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