Re: [PATCH v7 2/4] topology: use bin_attribute to break the size limitation of cpumap ABI

[Yury Norov]

On Mon, Jul 19, 2021 at 11:10:45AM +0000, Song Bao Hua (Barry Song) wrote:
> 
> 
> > -----Original Message-----
> > From: andriy.shevchenko@xxxxxxxxxxxxxxx
> > [mailto:andriy.shevchenko@xxxxxxxxxxxxxxx]
> > Sent: Monday, July 19, 2021 9:07 PM
> > To: Yury Norov <yury.norov@xxxxxxxxx>
> > Cc: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>;
> > gregkh@xxxxxxxxxxxxxxxxxxx; akpm@xxxxxxxxxxxxxxxxxxxx;
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> > Subject: Re: [PATCH v7 2/4] topology: use bin_attribute to break the size
> > limitation of cpumap ABI
> > 
> > On Fri, Jul 16, 2021 at 06:12:21PM -0700, Yury Norov wrote:
> > > On Sat, Jul 17, 2021 at 12:16:48AM +0000, Song Bao Hua (Barry Song) wrote:
> > > > > From: Yury Norov [mailto:yury.norov@xxxxxxxxx]
> > > > > Sent: Saturday, July 17, 2021 8:04 AM
> > > > > On Fri, Jul 16, 2021 at 08:49:58AM +0000, Song Bao Hua (Barry Song) wrote:
> > 
> > ...
> > 
> > > > Generally good idea. However, for sysfs ABI entries, it might not be
> > > > that true.
> > > >
> > > > A sysfs entry might never be read for its whole life. As I explained
> > > > before, a sysfs entry - especially for list,  is randomly "cat" by users.
> > > > Many of them won't be read forever. And after they are read once, they
> > > > will probably never be read again. The operations to read ABI could be
> > > > random and rare. Performance wouldn't be a concern.
> > > >
> > > > To avoid holding the memory which might never be used, it is better to
> > > > allocate and free the memory during runtime. I mean to allocate in show()
> > > > and free in show(), aka, to do it on demand.
> > > >
> > > > For example, for a server with 256CPU and each cpu has dozens of sysfs ABI
> > > > entries, only a few of sysfs list entries might be randomly "cat" by users.
> > > > Holding 256*entries memory doesn't look good.
> > >
> > > Ok, makes sense.
> > >
> > > > > This would require to add bitmap_max_string_size(list, bitmap, nbits),
> > > > > but it's O(1), and I think, others will find it helpful.
> > > >
> > > > What about getting size and memory at the same time?
> > >
> > > 1. We already have kasprintf()
> > > 2. It breaks coding style.
> > >
> > > Documentation/process/coding-style.rst:
> > >         Functions should be short and sweet, and do just one thing.
> > >
> > > From practical point of view, there should be some balance between
> > > granularity and ease-of-use. But in this case, bitmap_list cries for
> > > a function that will help to estimate size of output buffer.
> > 
> > According to the vsnprintf() logic the estimated size is what it returns. If
> > user supplies too few bytes available, the comparison with the returned value
> > can tell caller that space wasn't big enough.

snprintf(NULL, 0, "pbl", ...) also works, but it's O(nbits), and user is not
guaranteed that allocated memory would be always sufficient. I mean max possible
length for given nbits, not a length of a specific string.

In case of lists, the length may grow. Consider:
        0-8 -> 0-3,5-8 -> 0,2,4,6,8

If we want to allocate a storage for strings that may change, it would be
helpful to allocate memory for the lengthiest string in advance.

So, bitmap_max_string_len() may be a convenient O(1) alternative for
those who interested in printing the same bitmap in the same buffer.

> As far as my understanding, for estimated size in bitmap_max_string_size()
> Yury might mean something as below?
> 
> * For bitmask:
> Each 32bit needs 9 bytes "11111111,", so the max size of mask would be:
> 9*nr_cpus / 32 ?

11111 -> "f1", but your formula gives 1. 
I think it should be like this (not tested):
        DIV_ROUND_UP(nbits, 4) + nbits < 32 ? 0 : nbits / 32 - 1

> * For list:
> Maximally  cpu0-9 need 2bytes, like "1,"
> Maximally cpu10-99 need 3bytes, like "50,"
> Maximally cpu100-999 need 4bytes, like "101,"
> Maximally cpu1000-9999 need 5 bytes..
>  
> So once we know the size of the bitmap, we can figure out the maximum
> size of its string for mask and list?
> 
> Pls correct me if you don't mean this, yury.
 
Assuming that longest possible strings are of the form 0,2,4,6,... I
think it's correct except for the last comma, so substract 1.

If we decide to go on with this bitmap_max_strlen(), the list part
should be tested extensively.
 
> > > And it's
> > > easy to imagine a case where the estimated length of bitmap is needed
> > > explicitly:
> > >
> > >         size_t max_size = bitmap_max_string_size(nbits);
> > >         char *buf = kmalloc(PAGE_ALIGN(max_size) * nr_cpus);
> > >
> > > Thought, I don't insist. In your driver you can do:
> > >
> > >         size_t size = snprintf(NULL, 0, ...);
> > >         void *buf = kmalloc(size);
> > >
> > > It will be fully correct, and you already have everything you need.
> > >
> > > > ssize_t bitmap_get_print_buf(bool list, char **buf, const unsigned long
> > > >  *maskp, int nmaskbits)
> > > >
> > > > ssize_t cpumap_get_print_buf(bool list, char **buf, const struct cpumask
> > *mask);
> > > >
> > > > This API returns the size of printed buffer, and it also gets the
> > > > printed result saved in *buf. Then drivers don't need to do three
> > > > steps:
> > > >
> > > > 1. get cpumap buffer size which is your cpumap_max_string_size()
> > > > 2. allocate memory for buffer according to size got in step 1
> > > > 3. print bitmap(cpumap) to buffer by "pbl"
> > > >
> > > > It will only need to call bitmap_get_print_buf() and all three
> > > > things are done inside bitmap_get_print_buf().
> > > >
> > > > How to use the size and memory allocated in cpumap_get_print_buf
> > > > will be totally up to users.
> > > >
> > > > The other benefit for this is that if we get string size during initialization,
> > > > and then we print in show() entries, the size got at the beginning might
> > be not
> > > > enough as system topology might have changed. Sysfs ABI reflects the status
> > of
> > > > system at this moment.
> > 
> > --
> > With Best Regards,
> > Andy Shevchenko
> > 
> 
> Thanks
> Barry