Re: [PATCH] staging/lustre: use rcu_dereference to access rcu protected current->real_parent field

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, Aug 08, 2014 at 01:06:15AM -0400, Oleg Drokin wrote:
> 
> On Aug 8, 2014, at 12:42 AM, Greg Kroah-Hartman wrote:
> 
> > On Fri, Aug 08, 2014 at 12:03:20AM -0400, Oleg Drokin wrote:
> >> Hello!
> >> 
> >> On Aug 7, 2014, at 11:49 PM, Greg Kroah-Hartman wrote:
> >>>> 
> >>>> This is not a critical bug and in the worst case the code here may
> >>>> cause miss of statistics counter increase.
> >>>> This is why I think it is not worth to backport the patch at all.
> >>> You are right, and if this is just for some random "statistics" file,
> >>> can we just delete the whole function?
> >> 
> >> I hope not!
> >> This is used all around the client to tally up various operations executed counts.
> > Why would you do that?  Why would they care?
> 
> We would do that to provide information on the client operations performed.
> They would care because they are interested in what particular clients might be doing.
> 
> >> The statistic is then used by various userspace monitoring tools.
> > Why not use the in-kernel monitoring tools instead of creating your own?
> > What does userspace do with that information?
> 
> We don't really control the userspace tools. People write tools to suit their needs
> to monitor loads, see odd things the end users are doing or possibly for some
> debugging even.
> Correlating these numbers with what server sees also proves useful at times
> (write combining for example).
> 
> Here's a sample of output of a recently mounted client that I poked on a bit (the lines starting with # are my comments):
> # cat /proc/fs/lustre/llite/lustre-ffff88008dde27f0/stats
> snapshot_time             1407473168.466102 secs.usecs
> read_bytes                1 samples [bytes] 0 0 0
> write_bytes               4 samples [bytes] 2 7 19
> osc_write                 4 samples [bytes] 2 7 19
> # The bytes counts show you minimum, maximum of writes seen and total number of bytes read-written.
> # Lustre (and many other network filesystems) is very sensitive to small IO, esp. reads so it's good
> # to know if you have a lot of it.
> open                      6 samples [regs]
> # The "regs" type just shows you how many of given type operations were performed since last statistic reset.
> # Frequently that allows people to guess where does high load come from on a particular client when
> # it's otherwise not obvious because not a lot of cpu is used.
> # Some operations are heavier than others too.
> close                     6 samples [regs]
> readdir                   4 samples [regs]
> setattr                   1 samples [regs]
> truncate                  4 samples [regs]
> getattr                   7 samples [regs]
> create                    1 samples [regs]
> alloc_inode               1 samples [regs]
> getxattr                  8 samples [regs]
> inode_permission          28 samples [regs]
> 
> As more operations types are seen the list grows.
> Then there are also specific stats for readahead (data and metadata) so that interested people can make informed
> decisions on the tuning there should they be unsatisfied with default settings.
> 
> I am not sure there's a similar mechanism in the kernel already that
> would allow us to get this sort of data easily all in one place?

perf should show you this, if not, please add the functionality there.
A filesystem is not the place to have performance monitoring code, this
needs to be removed before it can be moved out of staging.  Please work
with the trace/perf developers on this if there is something lacking
there.

thanks,

greg k-h
dG

> 
> Bye,
>     Oleg