Re: [PATCH 2/2] refscale: prevent buffer to pr_alert() being too long

[Li, Zhijian]

on 2021/10/23 7:15, Paul E. McKenney wrote:
> On Fri, Oct 22, 2021 at 06:51:11PM +0800, Li Zhijian wrote:
> > 0Day/LKP observed that the refscale results become incompleted
> > when a larger nruns(such as 300) is specified.
> > It seems that printk() can accept < 1024 buffer at once.
> > Print the buffer if its length exceeds 800 simply.
> >
> > CC: Philip Li <philip.li@xxxxxxxxx>
> > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > Signed-off-by: Li Zhijian <lizhijian@xxxxxxxxxxxxxx>
> Good catch!  A couple of questions below.
>
> 						Thanx, Paul
>
> > ---
> >   kernel/rcu/refscale.c | 23 +++++++++++++----------
> >   1 file changed, 13 insertions(+), 10 deletions(-)
> >
> > diff --git a/kernel/rcu/refscale.c b/kernel/rcu/refscale.c
> > index 2cbe2a2ba387..b1b9052010fd 100644
> > --- a/kernel/rcu/refscale.c
> > +++ b/kernel/rcu/refscale.c
> > @@ -604,7 +604,7 @@ static u64 process_durations(int n)
> >   	char *buf;
> >   	u64 sum = 0;
> >   
> > -	buf = kmalloc(128 + nreaders * 32, GFP_KERNEL);
> > +	buf = kmalloc(64 * 20, GFP_KERNEL);
> This allocation (and the one below) is 1280 bytes rather than
> 1024 bytes.  Why the extra couple hundred bytes?

Nothing special, so let's change 1024 or (800 + 64 ),which is sufficent as well ?


> >   	if (!buf)
> >   		return 0;
> >   	buf[0] = 0;
> > @@ -617,13 +617,15 @@ static u64 process_durations(int n)
> >   
> >   		if (i % 5 == 0)
> >   			strcat(buf, "\n");
> > +		if (strlen(buf) > 800) {
> > +			pr_alert("%s", buf);
> Does the tools/testing/selftests/rcutorture/bin/kvm-recheck-refscale.sh
> script also require changes to handle the partial lines?

I missed that, i will take a look at it

Thanks

Zhijian


> Same for the later comparison against 800.
>
> > +			buf[0] = 0;
> > +		}
> >   		strcat(buf, buf1);
> >   
> >   		sum += rt->last_duration_ns;
> >   	}
> > -	strcat(buf, "\n");
> > -
> > -	SCALEOUT("%s\n", buf);
> > +	pr_alert("%s\n", buf);
> >   
> >   	kfree(buf);
> >   	return sum;
> > @@ -648,7 +650,7 @@ static int main_func(void *arg)
> >   
> >   	VERBOSE_SCALEOUT("main_func task started");
> >   	result_avg = kzalloc(nruns * sizeof(*result_avg), GFP_KERNEL);
> > -	buf = kzalloc(64 + nruns * 32, GFP_KERNEL);
> > +	buf = kzalloc(64 * 20, GFP_KERNEL);
> >   	if (!result_avg || !buf) {
> >   		VERBOSE_SCALEOUT_ERRSTRING("out of memory");
> >   		errexit = true;
> > @@ -701,10 +703,7 @@ static int main_func(void *arg)
> >   	if (errexit)
> >   		goto err;
> >   
> > -	buf[0] = 0;
> > -	strcat(buf, "\n");
> > -	strcat(buf, "Runs\tTime(ns)\n");
> > -
> > +	pr_alert("Runs\tTime(ns)\n");
> >   	for (exp = 0; exp < nruns; exp++) {
> >   		u64 avg;
> >   		u32 rem;
> > @@ -712,9 +711,13 @@ static int main_func(void *arg)
> >   		avg = div_u64_rem(result_avg[exp], 1000, &rem);
> >   		sprintf(buf1, "%d\t%llu.%03u\n", exp + 1, avg, rem);
> >   		strcat(buf, buf1);
> > +		if (strlen(buf) > 800) {
> > +			pr_alert("%s", buf);
> > +			buf[0] = 0;
> > +		}
> >   	}
> >   
> > -	SCALEOUT("%s", buf);
> > +	pr_alert("%s", buf);
> >   
> >   err:
> >   	// This will shutdown everything including us.
> > --
> > 2.33.0