linux-kernel - Re: [PATCH 1/2] test_xarray: add tests for advanced multi-index use

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Message-ID: <ZbQQXO5YhKhdr1Ou@bombadil.infradead.org>
Date: Fri, 26 Jan 2024 12:04:44 -0800
From: Luis Chamberlain <mcgrof@...nel.org>
To: Matthew Wilcox <willy@...radead.org>
Cc: kernel test robot <oliver.sang@...el.com>,
	Daniel Gomez <da.gomez@...sung.com>, oe-lkp@...ts.linux.dev,
	lkp@...el.com, linux-kernel@...r.kernel.org,
	"gost.dev@...sung.com" <gost.dev@...sung.com>,
	Pankaj Raghav <p.raghav@...sung.com>
Subject: Re: [PATCH 1/2] test_xarray: add tests for advanced multi-index use

On Fri, Jan 26, 2024 at 07:26:17PM +0000, Matthew Wilcox wrote:
> On Fri, Jan 26, 2024 at 11:12:03AM -0800, Luis Chamberlain wrote:
> > On Fri, Nov 17, 2023 at 01:01:18PM -0800, Luis Chamberlain wrote:
> > > On Fri, Nov 17, 2023 at 08:58:05PM +0000, Matthew Wilcox wrote:
> > > > On Fri, Nov 17, 2023 at 12:54:09PM -0800, Luis Chamberlain wrote:
> > > > > +/*
> > > > > + * Can be used in contexts which busy loop on large number of entries but can
> > > > > + * sleep and timing is if no importance to test correctness.
> > > > > + */
> > > > > +#define XA_BUG_ON_RELAX(xa, x) do {				\
> > > > > +	if ((tests_run % 1000) == 0)				\
> > > > > +		schedule();					\
> > > > > +	XA_BUG_ON(xa, x);					\
> > > > > +} while (0)
> > > > 
> > > > That is awful.  Please don't do that.  You're mixing two completely
> > > > unrelated thing into the same macro, which makes no sense.  Not only
> > > > that, it's a macro which refers to something in the containing
> > > > environment that isn't a paramter to the macro.
> > > 
> > > I figured you'd puke. Would you prefer I just open code the check on the loop
> > > though? I'm sure another alternative is we *not care* about these
> > > overloaded systems running the test. What would you prefer?
> > 
> > OK without any particular preferences outlined this is what I have,
> > splitting the two contexts and making the busy loop fix clearer.
> > 
> > +#define XA_BUSY_LOOP_RELAX(xa, x) do {                         \
> > +       if ((i % 1000) == 0)                                    \
> > +               schedule();                                     \
> > +} while (0)
> > +
> > +/*
> > + * Can be used in contexts which busy loop on large number of entries but can
> > + * sleep and timing is if no importance to test correctness.
> > + */
> > +#define XA_BUG_ON_RELAX(i, xa, x) do {                         \
> > +       XA_BUSY_LOOP_RELAX(i);                                  \
> > +       XA_BUG_ON(xa, x);                                       \
> > +} while (0)
> 
> No.  XA_BUG_ON_RELAX is not OK.  Really.
> 
> We have a perfectly good system for "relaxing":
> 
>         xas_for_each_marked(&xas, page, end, PAGECACHE_TAG_DIRTY) {
>                 xas_set_mark(&xas, PAGECACHE_TAG_TOWRITE);
>                 if (++tagged % XA_CHECK_SCHED)
>                         continue;
> 
>                 xas_pause(&xas);
>                 xas_unlock_irq(&xas);
>                 cond_resched();
>                 xas_lock_irq(&xas);
>         }

And yet we can get a soft lockup with order 20 (1,048,576 entries),
granted busy looping over 1 million entries is insane, but it seems it
the existing code may not be enough to avoid the soft lockup. Also
cond_resched() may be eventually removed [0].

Anyway, we can ignore the soft lockup then.

[0] https://lwn.net/Articles/950581/

  Luis