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

[Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>]

On Fri, Jul 24, 2020 at 05:34:11PM +0100, Chris Wilson wrote:
>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@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@intel.com>
>> >> Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@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

I see that the do_mmap()->mmap_region() takes a reference to file

vma->vm_file = get_file(file);

In our case this is perf_fd. do_munmap does a corresponding fput.

so unmap_mapping_range() is never called unless both unmap() and 
close(perf_fd) are called by user (or process terminates).

Is that good to take care of this ordering?

This also explains why I cannot get a VM_FAULT_SIGBUS with the IGTs.

>
>> >> +       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.

I will not use stream->oa_buffer.vma->obj in vm_fault_oa based on your 
earlier comments, so this should be taken care of.

Thanks,
Umesh

>-Chris
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