Hi Greg, Thanks for the comments, I agree with you on performance vs debugging code. However actual issue I want to stress on is iterator restarting multiple times and not the performance. thanks, srini On Fri, Jul 15, 2011 at 4:18 PM, Greg KH <gregkh@xxxxxxx> wrote: > > On Fri, Jul 15, 2011 at 09:51:32AM +0100, Srinivas KANDAGATLA wrote: > > Hi All, > > I have noticed that, reading or dumping a full sysfs entry (ex: > > /sys/kernel/debug/gpio ) > > That's not sysfs, it's debugfs. > > > which exceeds PAGE_SIZE restarts associated seq_file iterator > > (calling start->show->stop) > > morethan one time. > > > > In my case gpio entry just exceeded 8k which resulted in show > > function getting > > called 3 times. > > > > The reason is because the allocated buffer overflows and seq_file allocates > > new buffer of next order and starts iteration from the start, which > > is clear in > > seq_file code. > > However this is bit of conservative approach which results in: > > - restarting iterator if file sz exceeds PAGE_SIZE( which is really > > not necessary ) > > - not reusing the formated buffer (which can be done by re-alloc and copy). > > - Will help entires like dumping cacheline or pagetables to capture > > its state with better accuracy. > > > > To address this situation, I think seq_file formating/printf > > functions should > > re-allocate buffer as soon as they detect buffer overflow. > > This is debugging code, why should performance matter? > > > This patch attempts to do the same by adding new function > > buf_realloc which inturn > > calls krealloc and all the seq_file formating or printing functions > > call this > > function whenever an overflow is detected and retry. > > > > > > Main motive of this patch is to avoid restarting the iterator and NOT > > performance, however there is a bit of gain in performance too. > > > > Performance figures(average over 100 times loop) for different sizes of > > sysfs entries: > > > > without realloc: > > --------------------------------- > > SZ REAL USER SYS > > --------------------------------- > > | 4k | 0.012 0.002 0.009 > > | 8k | 0.013 0.002 0.010 > > | 16k | 0.015 0.003 0.012 > > | 32k | 0.019 0.002 0.017 > > -------------------------------- > > > > with realloc: > > --------------------------------- > > SZ REAL USER SYS > > --------------------------------- > > | 4k | 0.012 0.002 0.009 > > | 8k | 0.012 0.002 0.009 > > | 16k | 0.013 0.002 0.010 > > | 32k | 0.015 0.003 0.012 > > ------------------------------- > > > > > > I discovered this issue while I was working on a different issue as > > I happed > > to stick a printk in show function, which got called 3 times when I > > dumped the > > entry with 'cat' :-). which I did not expect. > > > > > > comments? :-) > > Again, debugfs doesn't matter for this type of thing, so adding > complexity to the core kernel code for it shouldn't be done unless > absolutely needed. > > thanks, > > greg k-h -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
