Re: [PATCH v2 3/3] hw_random: increase schedule timeout in rng_dev_read()

Amos Kong
2014-09-16T00:29:19Z

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On Mon, Sep 15, 2014 at 06:13:31PM +0200, Michael Büsch wrote:
> On Tue, 16 Sep 2014 00:02:29 +0800
> Amos Kong <akong@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@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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