[igt-dev] [PATCH i-g-t] lib/xe/xe_query: Extern xe_supports_faults()

[Matthew Brost]

On Mon, Mar 27, 2023 at 04:54:42PM -0700, Niranjana Vishwanathapura wrote:
> On Fri, Mar 24, 2023 at 10:21:51AM +0100, Zbigniew Kempczyński wrote:
> > On Thu, Mar 23, 2023 at 11:23:35PM -0700, Niranjana Vishwanathapura wrote:
> > > On Fri, Mar 24, 2023 at 07:12:46AM +0100, Mauro Carvalho Chehab wrote:
> > > > On Thu, 23 Mar 2023 22:02:53 -0700
> > > > Niranjana Vishwanathapura <niranjana.vishwanathapura at intel.com> wrote:
> > > >
> > > > > Do not check for supports_faults in xe_device_get() as
> > > > > it creates a VM in fault mode which prohibits creation
> > > > > of any other VM in non-fault mode until this fault mode
> > > > > VM is closed. This leads to test failures in multi threaded
> > > > > cases.
> > > >
> > > > Hmm...
> > > >
> > > > >
> > > > > Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura at intel.com>
> > > > > ---
> > > > >  lib/xe/xe_query.c | 51 ++++++++++++++++++++++-------------------------
> > > > >  lib/xe/xe_query.h |  3 ---
> > > > >  2 files changed, 24 insertions(+), 30 deletions(-)
> > > > >
> > > > > diff --git a/lib/xe/xe_query.c b/lib/xe/xe_query.c
> > > > > index 183523280..dc91d59bc 100644
> > > > > --- a/lib/xe/xe_query.c
> > > > > +++ b/lib/xe/xe_query.c
> > > > > @@ -160,23 +160,6 @@ static uint32_t __mem_default_alignment(struct drm_xe_query_mem_usage *mem_usage
> > > > >  	return alignment;
> > > > >  }
> > > > >
> > > > > -static bool xe_check_supports_faults(int fd)
> > > > > -{
> > > > > -	bool supports_faults;
> > > > > -
> > > > > -	struct drm_xe_vm_create create = {
> > > > > -		.flags = DRM_XE_VM_CREATE_ASYNC_BIND_OPS |
> > > > > -			 DRM_XE_VM_CREATE_FAULT_MODE,
> > > > > -	};
> > > > > -
> > > > > -	supports_faults = !igt_ioctl(fd, DRM_IOCTL_XE_VM_CREATE, &create);
> > > > > -
> > > > > -	if (supports_faults)
> > > > > -		xe_vm_destroy(fd, create.vm_id);
> > > >
> > > > Weren't the VM supposed to be closed here?
> > > >
> > > 
> > > Yes, but before it does destroy the VM, some other thread can try
> > > to create a VM in non-fault mode and fail because we have created
> > > a fault mode VM here. This happens in multi-threaded test like
> > > xe_exec_threads.
> > 
> > I've question about it - why separately created vm in fault mode
> > influences on creating another vm in non-fault mode? Those vm's
> > are separate entities so why they collide?
> > 
> > I've examined the code and at the moment I see two scenarios
> > - threads are reusing same xe_device instantiated on opening
> > fixture and are opening their own fd, what means each cached
> > xe_device entry reside on separate fd. I see there's lack
> > of proper locking during insertion to igt_map (I'm going to
> > send a fix in a minute). I bet this might be reason of problems
> > - multiple threads adding to hashmap (which might resize in
> > this moment).
> > 
> > I see there's risk of executing xe_check_supports_faults()
> > twice on same fd from two competing threads and this is not
> > mutexed. But create/destroy vm is on local stack and even with
> > this it shouldn't influence on other thread execution.
> > 
> 
> Zbigniew,
> 
> Looks like KMD Xe driver fault/non-fault mode is at device level
> (check xe_device_in_fault_mode(), it is used in couple places).
> 
> Probably Matt Brost can answer your question precisely.
>

Yes, the KMD check is at a device level, basically you can't use any
dma-fences if faults are used on another VM without the risk of
deadlocking, this is why we have these checks in the KMD,
 
> I wonder, whether we need this runtime switching between fault
> and non-fault modes. I would assume, we always use fault mode
> if device supports it, We can perhaps have a module load time
> parameter to force non-fault mode.
> 
> Matt, any thoughts?
>

I prefer to have this dynamic as we shouldn't force a paradigm on the
user.

> In any case, this patch seems good to me. It checks whether
> xe supports faults only for those tests that need it and also
> solves the problem we currently have. So, I suggest we go
> ahead with this patch. What do you think?
>

I agree this patch is good for now.

Matt

> Niranjana
> 
> > --
> > Zbigniew
> > 
> > > 
> > > Niranjana
> > > 
> > > 
> > > >
> > > > Regards,
> > > > Mauro