Linux Virtualization - Sep 2014 - [PATCH v2 3/3] hw_random: increase schedule timeout in rng_dev_read()

On Tue, 16 Sep 2014 00:02:29 +0800
Amos Kong <akong at redhat.com> wrote:
> This patch increases the schedule timeout to 10 jiffies, it's more
> appropriate, then other takes can easy to hold the mutex lock.
> 
> Signed-off-by: Amos Kong <akong at redhat.com>
> ---
>  drivers/char/hw_random/core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> index 263a370..b5d1b6f 100644
> --- a/drivers/char/hw_random/core.c
> +++ b/drivers/char/hw_random/core.c
> @@ -195,7 +195,7 @@ static ssize_t rng_dev_read(struct file *filp, char
__user *buf,
>  
>  		mutex_unlock(&rng_mutex);
>  
> -		schedule_timeout_interruptible(1);
> +		schedule_timeout_interruptible(10);
>  
>  		if (signal_pending(current)) {
>  			err = -ERESTARTSYS;
Does a schedule of 1 ms or 10 ms decrease the throughput?
I think we need some benchmarks.

-- 
Michael
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On Mon, Sep 15, 2014 at 06:13:31PM +0200, Michael B?sch
wrote:
> On Tue, 16 Sep 2014 00:02:29 +0800
> Amos Kong <akong at redhat.com> wrote:
> 
> > This patch increases the schedule timeout to 10 jiffies, it's more
> > appropriate, then other takes can easy to hold the mutex lock.
> > 
> > Signed-off-by: Amos Kong <akong at redhat.com>
> > ---
> >  drivers/char/hw_random/core.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/char/hw_random/core.c
b/drivers/char/hw_random/core.c
> > index 263a370..b5d1b6f 100644
> > --- a/drivers/char/hw_random/core.c
> > +++ b/drivers/char/hw_random/core.c
> > @@ -195,7 +195,7 @@ static ssize_t rng_dev_read(struct file *filp,
char __user *buf,
> >  
> >  		mutex_unlock(&rng_mutex);
> >  
> > -		schedule_timeout_interruptible(1);
> > +		schedule_timeout_interruptible(10);
> >  
> >  		if (signal_pending(current)) {
> >  			err = -ERESTARTSYS;
> 
> Does a schedule of 1 ms or 10 ms decrease the throughput?
In my test environment, 1 jiffe always works (100%), as suggested by
Amit 10 jiffes is more appropriate.

After applied current 3 patches, there is a throughput regression.

  1.2 M/s -> 6 K/s

We can only schedule in the end of loop (size == 0), and only for
non-smp guest. So smp guest won't be effected.

|               if (!size && num_online_cpus() == 1)
|                       schedule_timeout_interruptible(timeout);


Set timeout to 1:
  non-smp guest with quick backend (1.2M/s) -> about 49K/s)

Set timeout to 10:
  non-smp guest with quick backend (1.2M/s) -> about 490K/s)

We might need other benchmark to test the performance, but we can
see the bug clearly caused a regression.

As we discussed in other thread, need_resched() should work in this
case, so those patches might be wrong fixing.
> I think we need some benchmarks.
> 
> -- 
> Michael


-- 
			Amos.
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On Tue, 16 Sep 2014 08:27:40 +0800
Amos Kong <akong at redhat.com> wrote:
> Set timeout to 10:
>   non-smp guest with quick backend (1.2M/s) -> about 490K/s)
That sounds like an awful lot. This is a 60% loss in throughput.
I don't think we can live with that.

-- 
Michael
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