Re: [PATCH v2 17/26] selftests/resctrl: Replace file write with volatile variable

Ilpo Järvinen
2023-12-07T15:03:16Z

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On Tue, 28 Nov 2023, Reinette Chatre wrote:
> On 11/20/2023 3:13 AM, Ilpo Järvinen wrote:
> > The fill_buf code prevents compiler optimizating the entire read loop
> > away by writing the final value of the variable into a file. While it
> > achieves the goal, writing into a file requires significant amount of
> > work within the innermost test loop and also error handling.
> >
> > A simpler approach is to take advantage of volatile. Writing to a
> > variable through a volatile pointer is enough to prevent compiler from
> > optimizing the write away, and therefore compiler cannot remove the
> > read loop either.
> >
> > Add a volatile 'value_sink' into resctrl_tests.c and make fill_buf to
> > write into it. As a result, the error handling in fill_buf.c can be
> > simplified.
> >
>
> The subject and changelog describes the need for a volatile variable.
> The patch introduces two volatile variables. Could you please elaborate
> why two volatile variables are needed?

Well, the other "volatile variable" is a pointer to a volatile variable.

I've seen gcc to kill a static volatile int so I prefer to not take
change with its optimizer. Thus, I placed the sink into different
compilation unit and just present a pointer to the actual "volatile"
variable.

I guess the sink could be marked as plain int instead but this being
trickery to begin with I don't see much value either way. It's still
a trick.

I'll alter the changelog's wording though, "a volatile variable" is
not accurate as it's "a pointer to a volatile variable".

--
i.