RE: [PATCH v4 1/2] lib/test_bitops: Add benchmark test for fns()

[David Laight <David.Laight@ACULAB.COM>]

From: Yury Norov
> Sent: 01 May 2024 17:30
> 
> On Wed, May 01, 2024 at 09:20:46PM +0800, Kuan-Wei Chiu wrote:
> > Introduce a benchmark test for the fns(). It measures the total time
> > taken by fns() to process 1,000,000 test data generated using
> > get_random_bytes() for each n in the range [0, BITS_PER_LONG).
> >
> > example:
> > test_bitops: fns:          5876762553 ns, 64000000 iterations
> 
> So... 5 seconds for a test sounds too much. I see the following patch
> improves it dramatically, but in general let's stay in a range of
> milliseconds. On other machines it may run much slower and trigger
> a stall watchdog.
> 
> > Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
> 
> Suggested-by: Yury Norov <yury.norov@gmail.com>
> 
> > ---
> >
> > Changes in v4:
> > - Correct get_random_long() -> get_random_bytes() in the commit
> >   message.
> >
> >  lib/test_bitops.c | 22 ++++++++++++++++++++++
> >  1 file changed, 22 insertions(+)
> >
> > diff --git a/lib/test_bitops.c b/lib/test_bitops.c
> > index 3b7bcbee84db..ed939f124417 100644
> > --- a/lib/test_bitops.c
> > +++ b/lib/test_bitops.c
> > @@ -50,6 +50,26 @@ static unsigned long order_comb_long[][2] = {
> >  };
> >  #endif
> >
> > +static unsigned long buf[1000000];
> 
> Can you make it __init, or allocate with kmalloc_array(), so that 64M
> of memory will not last forever in the kernel?
> 
> > +static int __init test_fns(void)
> > +{
> > +	unsigned int i, n;
> > +	ktime_t time;
> > +
> > +	get_random_bytes(buf, sizeof(buf));
> > +	time = ktime_get();
> > +
> > +	for (n = 0; n < BITS_PER_LONG; n++)
> > +		for (i = 0; i < 1000000; i++)
> > +			fns(buf[i], n);
> 
> What concerns me here is that fns() is a in fact a const function, and
> the whole loop may be eliminated. Can you make sure it's not your case
> because 450x performance boost sounds a bit too much to me.
> 
> You can declare a "static volatile __used __init" variable to assign
> the result of fns(), and ensure that the code is not eliminated

Yep, without 'c' this compiler to 'return 0'.

static inline unsigned long fns(unsigned long word, unsigned int n)
{
	while (word && n--)
		word &= word - 1;
	return word ? __builtin_ffs(word) : 8 * sizeof (long);
}

unsigned long buf[1000000];

volatile int c;

int  test_fns(void)
{
	unsigned int i, n;

	for (n = 0; n < 8*sizeof (long); n++)
		for (i = 0; i < 1000000; i++)
			c = fns(buf[i], n);
	return 0;
}

You are also hitting the random number generator.
It would be better to use a predictable sequence.
Then you could, for instance, add up all the fns() results
and check you get the expected value.

With a really trivial 'RNG' (like step a CRC one bit) you
could do it inside the loop and not nee a buffer at all.

	David

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