Re: [PATCH v2] mm: vmalloc: Replace purge_lock spinlock with atomic refcount

[Joel Fernandes]

Hi Nick,

On Sun, Oct 16, 2016 at 9:00 PM, Nicholas Piggin  wrote:
> On Sat, 15 Oct 2016 03:42:42 -0700
> Joel Fernandes  wrote:
>
>> The purge_lock spinlock causes high latencies with non RT kernel. This has 
>> been
>> reported multiple times on lkml [1] [2] and affects applications like audio.
>>
>> In this patch, I replace the spinlock with an atomic refcount so that
>> preemption is kept turned on during purge. This Ok to do since [3] builds the
>> lazy free list in advance and atomically retrieves the list so any instance 
>> of
>> purge will have its own list it is purging. Since the individual vmap area
>> frees are themselves protected by a lock, this is Ok.
>
> This is a good idea, and good results, but that's not what the spinlock was
> for -- it was for enforcing the sync semantics.
>
> Going this route, you'll have to audit callers to expect changed behavior
> and change documentation of sync parameter.
>
> I suspect a better approach would be to instead use a mutex for this, and
> require that all sync=1 callers be able to sleep. I would say that most
> probably already can.

Thanks, I agree mutex is the right way to fix this.

Regards,
Joel

Re: [PATCH v2] mm: vmalloc: Replace purge_lock spinlock with atomic refcount

[Joel Fernandes]

Hi Nick,

On Sun, Oct 16, 2016 at 9:00 PM, Nicholas Piggin  wrote:
> On Sat, 15 Oct 2016 03:42:42 -0700
> Joel Fernandes  wrote:
>
>> The purge_lock spinlock causes high latencies with non RT kernel. This has 
>> been
>> reported multiple times on lkml [1] [2] and affects applications like audio.
>>
>> In this patch, I replace the spinlock with an atomic refcount so that
>> preemption is kept turned on during purge. This Ok to do since [3] builds the
>> lazy free list in advance and atomically retrieves the list so any instance 
>> of
>> purge will have its own list it is purging. Since the individual vmap area
>> frees are themselves protected by a lock, this is Ok.
>
> This is a good idea, and good results, but that's not what the spinlock was
> for -- it was for enforcing the sync semantics.
>
> Going this route, you'll have to audit callers to expect changed behavior
> and change documentation of sync parameter.
>
> I suspect a better approach would be to instead use a mutex for this, and
> require that all sync=1 callers be able to sleep. I would say that most
> probably already can.

Thanks, I agree mutex is the right way to fix this.

Regards,
Joel

Re: [PATCH v2] mm: vmalloc: Replace purge_lock spinlock with atomic refcount

[Nicholas Piggin]

On Sat, 15 Oct 2016 03:42:42 -0700
Joel Fernandes  wrote:

> The purge_lock spinlock causes high latencies with non RT kernel. This has 
> been
> reported multiple times on lkml [1] [2] and affects applications like audio.
> 
> In this patch, I replace the spinlock with an atomic refcount so that
> preemption is kept turned on during purge. This Ok to do since [3] builds the
> lazy free list in advance and atomically retrieves the list so any instance of
> purge will have its own list it is purging. Since the individual vmap area
> frees are themselves protected by a lock, this is Ok.

This is a good idea, and good results, but that's not what the spinlock was
for -- it was for enforcing the sync semantics.

Going this route, you'll have to audit callers to expect changed behavior
and change documentation of sync parameter.

I suspect a better approach would be to instead use a mutex for this, and
require that all sync=1 callers be able to sleep. I would say that most
probably already can.

Thanks,
Nick

Re: [PATCH v2] mm: vmalloc: Replace purge_lock spinlock with atomic refcount

[Nicholas Piggin]

On Sat, 15 Oct 2016 03:42:42 -0700
Joel Fernandes  wrote:

> The purge_lock spinlock causes high latencies with non RT kernel. This has 
> been
> reported multiple times on lkml [1] [2] and affects applications like audio.
> 
> In this patch, I replace the spinlock with an atomic refcount so that
> preemption is kept turned on during purge. This Ok to do since [3] builds the
> lazy free list in advance and atomically retrieves the list so any instance of
> purge will have its own list it is purging. Since the individual vmap area
> frees are themselves protected by a lock, this is Ok.

This is a good idea, and good results, but that's not what the spinlock was
for -- it was for enforcing the sync semantics.

Going this route, you'll have to audit callers to expect changed behavior
and change documentation of sync parameter.

I suspect a better approach would be to instead use a mutex for this, and
require that all sync=1 callers be able to sleep. I would say that most
probably already can.

Thanks,
Nick