linux-kernel - RE: [PATCH v2 17/24] selftests/resctrl: Replace count_bits with count_consecutive

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Date:   Thu, 1 Jun 2023 06:20:58 +0000
From:   "Shaopeng Tan (Fujitsu)" <tan.shaopeng@...itsu.com>
To:     'Ilpo Järvinen' 
        <ilpo.jarvinen@...ux.intel.com>
CC:     "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Shuah Khan <shuah@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2 17/24] selftests/resctrl: Replace count_bits with
 count_consecutive_bits()

Hi Ilpo,

> On Wed, 31 May 2023, Shaopeng Tan (Fujitsu) wrote:
> 
> > Hi Ilpo,
> >
> > > CAT and CMT tests depends on masks being continuous.
> > >
> > > Replace count_bits with more appropriate variant that counts consecutive
> bits.
> > >
> > > Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
> > > ---
> 
> > > @@ -218,6 +220,34 @@ static int get_bit_mask(char *filename,
> > > unsigned long
> > > *mask)
> > >  	return 0;
> > >  }
> > >
> > > +/*
> > > + * count_consecutive_bits - Returns the longest train of bits in a bit mask
> > > + * @val		A bit mask
> > > + * @start	The location of the least-significant bit of the longest train
> > > + *
> > > + * Return:	The length of the consecutive bits in the longest train of bits
> > > + */
> > > +unsigned int count_consecutive_bits(unsigned long val, unsigned int
> > > +*start) {
> > > +	unsigned long last_val;
> > > +	int count = 0;
> > > +
> > > +	while (val) {
> > > +		last_val = val;
> > > +		val &= (val >> 1);
> > > +		count++;
> > > +	}
> >
> > There may not be a case that the most-significant bit is 1 at present,
> > but if this case exists, it cannot count correctly.
> 
> Can you please rephrase what is your concern here please?
> 
> I test 0U, 1U, ~0U, and a few more complex combinations of bits, and all
> returned correct count so I might not have understood what case you meant
> with your description.
> 
> This function does not count nor calculate the most-significant bit in any
> phase but the longest train of bits using the count variable (and the
> least-significant bit of the longest train in the code that is not included into this
> partial snippet).

Thanks for your explanation.
I'm sorry, it was my mistake.
No problem here.

Best regards
Shaopeng TAN