linux-kernel - Re: [PATCH] lib/math/rational.c: Fix divide by zero

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Message-ID: <YKy9PHIbuhsomsTq@smile.fi.intel.com>
Date:   Tue, 25 May 2021 12:02:52 +0300
From:   Andy Shevchenko <andriy.shevchenko@...el.com>
To:     Trent Piepho <tpiepho@...il.com>
Cc:     Daniel Latypov <dlatypov@...gle.com>, linux-kernel@...r.kernel.org,
        andy@...nel.org, Andrew Morton <akpm@...ux-foundation.org>,
        Oskar Schirmer <oskar@...ra.com>, Yiyuan Guo <yguoaz@...il.com>
Subject: Re: [PATCH] lib/math/rational.c: Fix divide by zero

On Mon, May 24, 2021 at 01:17:48PM -0700, Trent Piepho wrote:
> On Mon, May 24, 2021 at 3:51 AM Andy Shevchenko
> <andriy.shevchenko@...el.com> wrote:
> > On Sat, May 22, 2021 at 05:18:06PM -0700, Trent Piepho wrote:
> >
> > This misses the test cases (*). Please, develop them with Daniel.
> >
> > *) We usually don't accept changes in the generic libraries without test cases.
> >
> > Fixes tag?
> 
> Is there a bug report on a tracker?  I just got the email from Yigua.

Fixes tag refers to the existing commit that brought the bug.
Also you may need to add Reported-by tag since Yigua reported it.

...

> > I think that refactoring may lead us to check first iteration before even going
> > into the loop. But it's another story and we may do it later (the algo uses
> 
> I started that, but it had no advantages and some disadvantages.
> 
> Basically, there are three cases: too large, too small & closest to
> zero, too small & closest to non-zero.  This code can handle those
> three cases by adding three branches, if(d1), if(n1), and if(!d1).
> The truth values we need already exist at this point the algorithm.
> 
> If it's at the start, then there still needs to be the three branches
> for each case.  But the values to test must be calculated too.
> 
> What's more, it's possible that the value is exactly representable in
> the allowed range.  That's actual appears to be the most common use
> case, reducing a fraction to lowest terms (*).  By putting the tests
> in the "terminate because of limits" case, they don't need to happen
> when "terminate because exact value find" is the result. If the check
> was first, then it would always happen, even if it wouldn't have been
> necessary.
> 
> And the time it took to find this bug shows us that out of bounds
> inputs are not a common case, so putting that on the hot path by
> checking it first at the expense of the reducing to lowest terms path
> doesn't make sense.

Thanks for detailed explanation of your view to the current state of the code.
As you noticed I am not insisting on refactoring or so, I was rather wondering
if it can be done in the future. Still we might need some performance tests.

Daniel, does KUnit have a capability to test performance?
Like running test case 1M times or so and calc average (median?) time of
execution.

> (*)  One could write a reduce to lowest terms function with an easier
> interface.  It could be a trivial one expression wrapper around
> rational_best_approximation().  It could also be a simpler function,
> but I think it would still perform the exact same sequence of
> divisions and moduli, so it wouldn't really make any difference.

-- 
With Best Regards,
Andy Shevchenko