Re: [Intel-gfx] [RFC 5/8] drm/i915: Contexts can use struct pid stored in the client

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 10/01/2020 13:42, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2020-01-10 13:30:46)
>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>
>> Now that contexts keep their parent client reference counted, we can
>> remove separate struct pid reference owned by contexts in favour of the
>> one already held by the client.
>>
>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>> ---
>>   drivers/gpu/drm/i915/gem/i915_gem_context.c       | 13 ++++---------
>>   drivers/gpu/drm/i915/gem/i915_gem_context_types.h | 10 ----------
>>   drivers/gpu/drm/i915/i915_debugfs.c               |  7 ++++---
>>   drivers/gpu/drm/i915/i915_gpu_error.c             |  8 ++++----
>>   4 files changed, 12 insertions(+), 26 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
>> index ba8ccc754f20..758cebb99ba4 100644
>> --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
>> @@ -323,7 +323,6 @@ static void i915_gem_context_free(struct i915_gem_context *ctx)
>>          if (ctx->timeline)
>>                  intel_timeline_put(ctx->timeline);
>>   
>> -       put_pid(ctx->pid);
>>          mutex_destroy(&ctx->mutex);
>>   
>>          kfree_rcu(ctx, rcu);
>> @@ -794,24 +793,20 @@ static int gem_context_register(struct i915_gem_context *ctx,
>>                  WRITE_ONCE(vm->file, fpriv); /* XXX */
>>          mutex_unlock(&ctx->mutex);
>>   
>> -       ctx->pid = get_task_pid(current, PIDTYPE_PID);
>>          snprintf(ctx->name, sizeof(ctx->name), "%s[%d]",
>> -                current->comm, pid_nr(ctx->pid));
>> +                current->comm, pid_nr(client->pid));
>>   
>>          /* And finally expose ourselves to userspace via the idr */
>>          ret = xa_alloc(&fpriv->context_xa, id, ctx, xa_limit_32b, GFP_KERNEL);
>> -       if (ret) {
>> -               put_pid(fetch_and_zero(&ctx->pid));
>> -               goto out;
>> -       }
>> +       if (ret)
>> +               return ret;
>>   
>>          ctx->client = client = i915_drm_client_get(fpriv->client);
>>          spin_lock(&client->ctx_lock);
>>          list_add_tail_rcu(&ctx->client_link, &client->ctx_list);
>>          spin_unlock(&client->ctx_lock);
>>   
>> -out:
>> -       return ret;
>> +       return 0;
>>   }
>>   
>>   int i915_gem_context_open(struct drm_i915_private *i915,
>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context_types.h b/drivers/gpu/drm/i915/gem/i915_gem_context_types.h
>> index 879824159646..23421377a43d 100644
>> --- a/drivers/gpu/drm/i915/gem/i915_gem_context_types.h
>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_context_types.h
>> @@ -90,16 +90,6 @@ struct i915_gem_context {
>>           */
>>          struct i915_address_space __rcu *vm;
>>   
>> -       /**
>> -        * @pid: process id of creator
>> -        *
>> -        * Note that who created the context may not be the principle user,
>> -        * as the context may be shared across a local socket. However,
>> -        * that should only affect the default context, all contexts created
>> -        * explicitly by the client are expected to be isolated.
>> -        */
>> -       struct pid *pid;
>> -
>>          /** link: place with &drm_i915_private.context_list */
>>          struct list_head link;
>>          struct llist_node free_link;
>> diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
>> index 8f01c2bc7355..bc533501b4e0 100644
>> --- a/drivers/gpu/drm/i915/i915_debugfs.c
>> +++ b/drivers/gpu/drm/i915/i915_debugfs.c
>> @@ -346,7 +346,8 @@ static void print_context_stats(struct seq_file *m,
>>                          rcu_read_unlock();
>>   
>>                          rcu_read_lock();
>> -                       task = pid_task(ctx->pid ?: file->pid, PIDTYPE_PID);
>> +                       task = pid_task(ctx->client->pid ?: file->pid,
>> +                                       PIDTYPE_PID);
>>                          snprintf(name, sizeof(name), "%s",
>>                                   task ? task->comm : "<unknown>");
>>                          rcu_read_unlock();
>> @@ -1492,10 +1493,10 @@ static int i915_context_status(struct seq_file *m, void *unused)
>>                  spin_unlock(&i915->gem.contexts.lock);
>>   
>>                  seq_puts(m, "HW context ");
>> -               if (ctx->pid) {
>> +               if (ctx->client->pid) {
>>                          struct task_struct *task;
>>   
>> -                       task = get_pid_task(ctx->pid, PIDTYPE_PID);
>> +                       task = get_pid_task(ctx->client->pid, PIDTYPE_PID);
>>                          if (task) {
>>                                  seq_printf(m, "(%s [%d]) ",
>>                                             task->comm, task->pid);
>> diff --git a/drivers/gpu/drm/i915/i915_gpu_error.c b/drivers/gpu/drm/i915/i915_gpu_error.c
>> index fda0977d2059..9240327bdb7d 100644
>> --- a/drivers/gpu/drm/i915/i915_gpu_error.c
>> +++ b/drivers/gpu/drm/i915/i915_gpu_error.c
>> @@ -1235,8 +1235,8 @@ static void record_request(const struct i915_request *request,
>>          erq->pid = 0;
>>          rcu_read_lock();
>>          ctx = rcu_dereference(request->context->gem_context);
>> -       if (ctx)
>> -               erq->pid = pid_nr(ctx->pid);
>> +       if (ctx && ctx->client->pid)
>> +               erq->pid = pid_nr(ctx->client->pid);
>>          rcu_read_unlock();
>>   }
>>   
>> @@ -1313,11 +1313,11 @@ static bool record_context(struct drm_i915_error_context *e,
>>          if (ctx && !kref_get_unless_zero(&ctx->ref))
>>                  ctx = NULL;
>>          rcu_read_unlock();
>> -       if (!ctx)
>> +       if (!ctx || !ctx->client->pid)
>>                  return false;
> 
> Can we have a client without a pid?

Yes, when fbdev registers I ignore it as a client because sysfs is not 
there yet. I contemplated stashing it somewhere and registering later 
but did not yet decide to actually do it. This was actually because 
without sysfs I had no use for pid and name. Perhaps now that I have 
moved this patch earlier I can change this to always have those two.. 
okay I'll see.

> 
> Holding a reference to the ctx is sufficent to protect the
> i915_drm_client, ok.
> 
>>          rcu_read_lock();
>> -       task = pid_task(ctx->pid, PIDTYPE_PID);
>> +       task = pid_task(ctx->client->pid, PIDTYPE_PID);
>>          if (task) {
>>                  strcpy(e->comm, task->comm);
>>                  e->pid = task->pid;
> 
> Ok, if you move this ahead to patch 3, we can use this as justification
> for i915_drm_client all by itself.

Yes mostly done but will see the above, plus a small regression in 
busyness tracking crept in after rebase, somehow. Will post the series 
when I resolve that.

Regards,

Tvrtko

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