Re: [PATCH v7 4/4] lib: test_bitmap: add bitmap_print_to_buf test cases

[Yury Norov <yury.norov@gmail.com>]

On Thu, Jul 22, 2021 at 07:47:28PM +0200, Greg Kroah-Hartman wrote:
> On Thu, Jul 22, 2021 at 10:09:27AM -0700, Yury Norov wrote:
> > On Wed, Jul 21, 2021 at 01:40:36PM +0200, Greg Kroah-Hartman wrote:
> > > On Thu, Jul 15, 2021 at 04:23:32PM -0700, Yury Norov wrote:
> > > > On Fri, Jul 16, 2021 at 12:32:45AM +0300, Andy Shevchenko wrote:
> > > > > On Thu, Jul 15, 2021 at 11:48 PM Yury Norov <yury.norov@gmail.com> wrote:
> > > > > > On Thu, Jul 15, 2021 at 03:09:39PM +0300, Andy Shevchenko wrote:
> > > > > > > On Thu, Jul 15, 2021 at 11:58:56PM +1200, Barry Song wrote:
> > > > > > > > The added test items cover both cases where bitmap buf of the printed
> > > > > > > > result is greater than and less than 4KB.
> > > > > > > > And it also covers the case where offset for bitmap_print_to_buf is
> > > > > > > > non-zero which will happen when printed buf is larger than one page
> > > > > > > > in sysfs bin_attribute.
> > > > > > >
> > > > > > > More test cases is always a good thing, thanks!
> > > > > >
> > > > > > Generally yes. But in this case... I believe, Barry didn't write that
> > > > > > huge line below by himself. Most probably he copy-pasted the output of
> > > > > > his bitmap_print_buf() into the test. If so, this code tests nothing,
> > > > > > and just enforces current behavior of snprintf.
> > > > > 
> > > > > I'm not sure I got what you are telling me. The big line is to test
> > > > > strings that are bigger than 4k.
> > > > 
> > > > I'm trying to say that human are not able to verify correctness of
> > > > this line. The test is supposed to check bitmap_print_to_buf(), but
> > > > reference output itself is generated by bitmap_print_to_buf(). This
> > > > test will always pass by design, even if there's an error somewhere
> > > > in the middle, isn't it?
> > > 
> > > Then please manually check it to verify it is correct or not.  Once we
> > > have it verified, that's fine, it will remain static in this test for
> > > always going forward.
> > > 
> > > That's what "oracles" are for, there is nothing wrong with this test
> > > case or "proof" that I can see.
> > > 
> > > > > 
> > > > > ...
> > > > > 
> > > > > > > > +static const char large_list[] __initconst = /* more than 4KB */
> > > > > > > > +   "0,4,8,12,16,20,24,28,32-33,36-37,40-41,44-45,48-49,52-53,56-57,60-61,64,68,72,76,80,84,88,92,96-97,100-101,104-1"
> > > > > > > > +   "05,108-109,112-113,116-117,120-121,124-125,128,132,136,140,144,148,152,156,160-161,164-165,168-169,172-173,176-1"
> > > > > > > > +   "77,180-181,184-185,188-189,192,196,200,204,208,212,216,220,224-225,228-229,232-233,236-237,240-241,244-245,248-2"
> > > > > >
> > > > > > I don't like this behavior of the code: each individual line is not a
> > > > > > valid bitmap_list. I would prefer to split original bitmap and print
> > > > > > list representation of parts in a compatible format; considering a
> > > > > > receiving part of this splitting machinery.
> > > > > 
> > > > > I agree that split is not the best here, but after all it's only 1
> > > > > line and this is on purpose.
> > > > 
> > > > What I see is that bitmap_print_to_buf() is called many times,
> > > 
> > > That is not what the above list shows at all, it's one long string all
> > > together, only split up to make it easier for us to work with.
> > > 
> > > > and
> > > > each time it returns something that is not a valid bitmap list string.
> > > > If the caller was be able to concatenate all the lines returned by
> > > > bitmap_print_to_buf(), he'd probably get correct result. But in such
> > > > case, why don't he use scnprintf("pbl") directly?
> > > 
> > > I do not understand the objection here at all.  This series is fixing a
> > > real problem that eeople are having
> > 
> > I explicitly asked about an example of this problem. Barry answered in
> > a great length, but the key points are:
> > 
> >         https://lore.kernel.org/lkml/4ab928f1fb3e4420974dfafe4b32f5b7@hisilicon.com/
> > 
> >         > > So, the root problem here is that some machines have so many CPUs that
> >         > > their cpumask text representation may not fit into the full page in the
> >         > > worst case. Is my understanding correct? Can you share an example of
> >         > > such configuration?
> >         > 
> >         > in my understanding, I have not found any machine which has really
> >         > caused the problem till now.
> > 
> >         > [...]
> >         > 
> >         > This doesn't really happen nowadays as the maximum
> >         > NR_CPUS is 8196 for X86_64 and 4096 for ARM64 since 8196 * 9 / 32 = 2305
> >         > is still smaller than 4KB page size.
> > 
> > 
> > If it's not true, can you or Barry please share such an example?
> 
> So for a 4k page size, if you have every-other-cpu-enabled on x86, it
> will overflow this, right?
> 
> And I have heard of systems much bigger than this as well.  Why do you
> not think that large number of CPUs are not around?

I asked a question: is it urgent?, and I've got an answer: not urgent.
 
> > > and your complaining about test
> > > strings is _VERY_ odd.
> > 
> > The test itself is bad, but it's a minor issue.
> > 
> > My main complain is that the bitmap part of this series introduces a
> > function that requires O(N^2) of CPU time and O(N) of memory to just
> > print a string. The existing snprintf does this in O(N) and O(1)
> > respectively. Additionally to that, the proposed function has some
> > flaws in design.
> 
> Can you propose a better solution?

Yes. Fix sysfs to let bin_attr store a pointer to relevant data.
Meanwhile, use this O(N^2) hack locally.

> And is O(N^2) even an issue for this?

If it's in lib/bitmap than yes, because it's exposed to the whole
kernel.

Thanks,
Yury