Re: [LTP] [PATCH v2] madvise06: shrink to 3 MADV_WILLNEED pages to stabilize the test

[Richard Palethorpe <rpalethorpe@suse.de>]

Hello Li,

Li Wang <liwang@redhat.com> writes:

> Paul Bunyan reports that the madvise06 test fails intermittently with many
> LTS kernels, after checking with mm developer we prefer to think this is
> more like a test issue (but not kernel bug):
>
>    madvise06.c:231: TFAIL: 4 pages were faulted out of 2 max
>
> So this improvement is target to reduce the false positive happens from
> three points:
>
>   1. Adding the while-loop to give more chances for madvise_willneed()
>      reads memory asynchronously
>   2. Raise value of `loop` to let test waiting for more times if swapchache
>      haven't reached the expected
>   3. Shrink to only 3 pages for verifying MADV_WILLNEED that to make the
>      system easily takes effect on it
>
> From Rafael Aquini:
>
>   The problem here is that MADV_WILLNEED is an asynchronous non-blocking
>   hint, which will tell the kernel to start doing read-ahead work for the
>   hinted memory chunk, but will not wait up for the read-ahead to finish.
>   So, it is possible that when the dirty_pages() call start re-dirtying
>   the pages in that target area, is racing against a scheduled swap-in
>   read-ahead that hasn't yet finished. Expecting faulting only 2 pages
>   out of 102400 also seems too strict for a PASS threshold.
>
> Note:
>   As Rafael suggested, another possible approach to tackle this failure
>   is to tally up, and loosen the threshold to more than 2 major faults
>   after a call to madvise() with MADV_WILLNEED.
>   But from my test, seems the faulted-out page shows a significant
>   variance in different platforms, so I didn't take this way.
>
> Btw, this patch get passed on my two easy reproducible systems more than 1000 times
>
> Reported-by: Paul Bunyan <pbunyan@redhat.com>
> Signed-off-by: Li Wang <liwang@redhat.com>
> Cc: Rafael Aquini <aquini@redhat.com>
> Cc: Richard Palethorpe <rpalethorpe@suse.com>

Reviewed-by: Richard Palethorpe <rpalethorpe@suse.com>

-- 
Thank you,
Richard.

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