Re: [LTP] [PATCH v4 1/3] mm/thp: new case thp04.c

[chrubis@suse.cz]

Hi!
> >> +		 * or not.
> >> +		 */
> >> +		sleep(10);
> >> +		count++;
> >> +
> >> +		SAFE_FILE_SCANF(cleanup, PATH_KHPD "pages_collapsed",
> >> +			       "%ld", &pages_collapsed);
> >> +		SAFE_FILE_SCANF(cleanup, PATH_KHPD "max_ptes_none",
> >> +			       "%ld", &max_ptes_none);
> >> +		SAFE_FILE_SCANF(cleanup, PATH_KHPD "pages_to_scan",
> >> +			       "%ld", &pages_to_scan);
> >> +
> >> +		if (pages_collapsed != old_pages_collapsed ||
> >> +		    max_ptes_none != old_max_ptes_none ||
> >> +		    pages_to_scan != old_pages_to_scan) {
> >> +			old_pages_collapsed = pages_collapsed;
> >> +			old_max_ptes_none = max_ptes_none;
> >> +			old_pages_to_scan = pages_to_scan;
> >> +		} else {
> >> +			changing = 0;
> >> +		}
> >> +	}
> >> +
> >> +	tst_resm(TINFO, "khugepaged daemon takes %ds to scan all thp pages",
> >> +		 count * 10);
> >> +}
> > Ah, so it acutally does polling, this is fine.
> >
> > How long scanning takes. If it is about 10 seconds, we should poll
> > faster so that the testcase has chance to finish reasonably fast.
> > (if it takes 10 seconds and we miss the first windows it will sleep
> >   doing noting for another ten seconds)
> 
> so how about 5 seconds? I will test the value on different systems.

If the scan takes 10 seconds to finish I would go for 2 seconds sleep as
that can increase the duration maximally by 20%, which is not as bad as
100% or 50%. The overhead of opening and reading a file should be really
small.

> >> +	write_file(path, "always");
> > Where this came from? Does the function check for failures? Why not to
> > use SAFE_FILE_PRINTF() as the rest of the file?
> 
> yes, there's some confused issue here.
> 
> but I have tried to use SAFE_FILE_PRINTF, but it didn't work fine for 
> transparent_hugepage/enabled,
> if # cat /sys/kernel/mm/transparent_hugepage/enabled
>        always [madvise] never
> SAFE_FILE_PRINTF only get 'always', expected 'madvise'
> if # cat /sys/kernel/mm/transparent_hugepage/enabled
> [always] madvise never
> SAFE_FILE_PRINTF get '[always]', expected 'always'
> 
> I didn't find the reason why SAFE_FILE_PRINTF couldn't get the expected 
> value?

That is strange, the only difference is that SAFE_FILE_PRINTF() uses
FILE interface while write_file() uses file descriptor directly.
Otherwise they should do exactly the same in this case. If they don't
something is broken.

Do you have a snipped of code that misbehaves this way?

-- 
Cyril Hrubis
chrubis@suse.cz

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