[Intel-gfx] [PATCH 4/4] drm/i915/perf: Map OA buffer to user space for gen12 performance query

[Chris Wilson]

Quoting Umesh Nerlige Ramappa (2020-07-24 17:29:56)
> On Fri, Jul 24, 2020 at 01:42:33PM +0100, Chris Wilson wrote:
> >Quoting Umesh Nerlige Ramappa (2020-07-24 01:19:01)
> >> From: Piotr Maciejewski <piotr.maciejewski at intel.com>
> >>
> >> i915 used to support time based sampling mode which is good for overall
> >> system monitoring, but is not enough for query mode used to measure a
> >> single draw call or dispatch. Gen9-Gen11 are using current i915 perf
> >> implementation for query, but Gen12+ requires a new approach for query
> >> based on triggered reports within oa buffer.
> >>
> >> Triggering reports into the OA buffer is achieved by writing into a
> >> a trigger register. Optionally an unused counter/register is set with a
> >> marker value such that a triggered report can be identified in the OA
> >> buffer. Reports are usually triggered at the start and end of work that
> >> is measured.
> >>
> >> Since OA buffer is large and queries can be frequent, an efficient way
> >> to look for triggered reports is required. By knowing the current head
> >> and tail offsets into the OA buffer, it is easier to determine the
> >> locality of the reports of interest.
> >>
> >> Current perf OA interface does not expose head/tail information to the
> >> user and it filters out invalid reports before sending data to user.
> >> Also considering limited size of user buffer used during a query,
> >> creating a 1:1 copy of the OA buffer at the user space added undesired
> >> complexity.
> >>
> >> The solution was to map the OA buffer to user space provided
> >>
> >> (1) that it is accessed from a privileged user.
> >> (2) OA report filtering is not used.
> >>
> >> These 2 conditions would satisfy the safety criteria that the current
> >> perf interface addresses.
> >>
> >> To enable the query:
> >> - Add an ioctl to expose head and tail to the user
> >> - Add an ioctl to return size and offset of the OA buffer
> >> - Map the OA buffer to the user space
> >>
> >> v2:
> >> - Improve commit message (Chris)
> >> - Do not mmap based on gem object filp. Instead, use perf_fd and support
> >>   mmap syscall (Chris)
> >> - Pass non-zero offset in mmap to enforce the right object is
> >>   mapped (Chris)
> >> - Do not expose gpu_address (Chris)
> >> - Verify start and length of vma for page alignment (Lionel)
> >> - Move SQNTL config out (Lionel)
> >>
> >> v3: (Chris)
> >> - Omit redundant checks
> >> - Return VM_FAULT_SIGBUS is old stream is closed
> >> - Maintain reference counts to stream in vm_open and vm_close
> >> - Use switch to identify object to be mapped
> >>
> >> v4: Call kref_put on closing perf fd (Chris)
> >> v5:
> >> - Strip access to OA buffer from unprivileged child of a privileged
> >>   parent. Use VM_DONTCOPY
> >> - Enforce MAP_PRIVATE by checking for VM_MAYSHARE
> >>
> >> Signed-off-by: Piotr Maciejewski <piotr.maciejewski at intel.com>
> >> Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa at intel.com>
> >> ---
> >> @@ -3314,12 +3427,113 @@ static int i915_perf_release(struct inode *inode, struct file *file)
> >>         i915_perf_destroy_locked(stream);
> >>         mutex_unlock(&perf->lock);
> >>
> >> +       unmap_mapping_range(file->f_mapping, 0, OA_BUFFER_SIZE, 1);
> >
> >You can just used unmap_mapping_range(file->f_mapping, 0, -1, 1);
> >It scales with the number of vma present, so no worries, be conservative.
> >(Otherwise, you need s/0/OA_BUFFER_OFFSET/.)
> >
> >> +
> >>         /* Release the reference the perf stream kept on the driver. */
> >>         drm_dev_put(&perf->i915->drm);
> >>
> >>         return 0;
> >>  }
> >>
> >> +static void vm_open_oa(struct vm_area_struct *vma)
> >> +{
> >> +       struct i915_perf_stream *stream = vma->vm_private_data;
> >> +
> >> +       GEM_BUG_ON(!stream);
> >> +       perf_stream_get(stream);
> >> +}
> >> +
> >> +static void vm_close_oa(struct vm_area_struct *vma)
> >> +{
> >> +       struct i915_perf_stream *stream = vma->vm_private_data;
> >> +
> >> +       GEM_BUG_ON(!stream);
> >> +       perf_stream_put(stream);
> >> +}
> >> +
> >> +static vm_fault_t vm_fault_oa(struct vm_fault *vmf)
> >> +{
> >> +       struct vm_area_struct *vma = vmf->vma;
> >> +       struct i915_perf_stream *stream = vma->vm_private_data;
> >> +       struct i915_perf *perf = stream->perf;
> >> +       struct drm_i915_gem_object *obj = stream->oa_buffer.vma->obj;
> >> +       int err;
> >> +       bool closed;
> >
> >So vm_area_struct has a reference to the stream, that looks good now.
> >But there's no reference held to the vma itself.
> 
> How do I get a reference to the vma.

That would be i915_vma_get(), but you don't need to if we control the
order correctly, as then neither the PTE nor the ongoing faulthandler
last longer than the i915_perf_stream
 
> >> +       mutex_lock(&perf->lock);
> >> +       closed = READ_ONCE(stream->closed);
> >> +       mutex_unlock(&perf->lock);
> >
> >We do WRITE_ONCE(stream->closed, true) then invalidate all the mappings,
> >so that part looks good. The invalidate is serialised with the
> >vm_fault_oa, so we can just use a plain READ_ONCE(stream->closed) here
> >and not worry about the perf->lock.
> 
> will do
> >
> >However... I think it should close&invalidate before releasing
> >stream->oa_buffer.
> 
> will do
> >
> >And the read here of stream->oa_buffer should be after checking
> >stream->closed.
> 
> I don't understand. I am checking for closed before remap_io_sg.

It's the 

struct drm_i915_gem_object *obj = stream->oa_buffer.vma->obj;

that's before the stream->closed check. That's dereferencing vma, but vma
will be set to NULL in i915_perf_destroy.
-Chris
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