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Date:   Thu, 22 Jul 2021 20:45:25 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Stefano Brivio <sbrivio@...hat.com>,
        "Ma, Jianpeng" <jianpeng.ma@...el.com>,
        Valentin Schneider <valentin.schneider@....com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Guodong Xu <guodong.xu@...aro.org>, tangchengchang@...wei.com,
        "Zengtao (B)" <prime.zeng@...ilicon.com>,
        yangyicong <yangyicong@...wei.com>, tim.c.chen@...ux.intel.com,
        Linuxarm <linuxarm@...wei.com>
Subject: Re: [PATCH v7 4/4] lib: test_bitmap: add bitmap_print_to_buf test
 cases

On Thu, Jul 22, 2021 at 11:27:05AM -0700, Yury Norov wrote:
> 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@...il.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.

Just because people can not publically state that they are running Linux
on bigger boxes than this, does NOT mean that they are not running Linux
on bigger boxes than this.

So sometimes, you just have to trust that if someone says "hey, this is
a problem over here", that maybe it really is a problem.

> > > > 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.

Why?  It changes all the time and should be generated dynamically.

> Meanwhile, use this O(N^2) hack locally.

Who else uses this that it matters?

> > 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.

Who else will use it that it matters for speed?

And did you measure the speed of this?

thanks,

greg k-h

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