On 09/12/2013 01:30 PM, Linus Torvalds wrote:
On Thu, Sep 12, 2013 at 9:38 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
On Thu, Sep 12, 2013 at 7:55 AM, Waiman Long<Waiman.Long@xxxxxx> wrote:
Change log
----------
v1->v2:
- Rename the new seqlock primitives to read_seqexcl_lock* and
read_seqexcl_unlock*.
Applied.
Btw, when I tried to benchmark this, I failed miserably.
Why?
This patch is just a safety guard to prevent occasional bad performance
because of some bad timing. It will not improve performance for many
cases because the seqbegin/seqretry sequence succeeds without actual retry.
If you do a threaded benchmark of "getcwd()", you end up spending all
your time in a spinlock anyway: get_fs_root_and_pwd() takes the
fs->lock to get the root/pwd.
I am aware that there is another spinlock bottleneck in the fs struct
for getcwd().
Now, AIM7 probably uses processes, not threads, so you don't see this,
and maybe I shouldn't care. But looking at it, it annoys me
enormously, because the whole get_fs_root_and_pwd() is just stupid.
AIM7 don't do much getcwd() calls, so it is not a real bottleneck for
the benchmark. The lockref patch boosts the short workload performance.
The prepend_path patch was to fix the incorrect perf record data as
perf makes heavy use of d_path(). The change made to getcwd() was just a
side benefit. But then it still have other spinlock bottleneck.
Putting it all under the RCU lock and then changing it to use
get_fs_root_and_pwd_rcu() that just uses the fs->seq sequence
read-lock looks absolutely trivial.
Yes, I think we can do something similar for this. I will take a look to
see how it can be fixed.
-Longman
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