Messages in this thread | | | Date | Fri, 23 Mar 2012 14:25:50 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [GIT PULL] RCU changes for v3.4 |
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On Fri, Mar 23, 2012 at 02:16:38PM -0700, Paul E. McKenney wrote: > On Fri, Mar 23, 2012 at 12:39:59PM -0700, Linus Torvalds wrote: > > On Fri, Mar 23, 2012 at 12:21 PM, Paul E. McKenney > > <paulmck@linux.vnet.ibm.com> wrote: > > >> > > >> Please? Every time I look at some profiles, that silly rcu_read_lock > > >> is there in the profile. It's annoying. I'd rather see it in the > > >> function that invokes it. > > > > > > Let me see what I can do... > > > > Thanks. To some degree, rcu_read_lock() is the more critical one, > > because it is often in the much more critical path in the caller. In > > particular, it's often at the beginning of a function, where a number > > of arguments are "live", and calling it out-of-line also forces the > > compiler to then save/restore those arguments (because they are > > clobbered by the function call). > > > > rcu_read_unlock() is *usually* not as critical, and is obviously much > > harder to inline anyway due to the whole complexity with needing to > > check if an RCU sequence has ended. It often is at the end of the > > function call in the caller, when the only thing like is often just > > the single return value (if that). So it seldom looks nearly as bad in > > any profiles, because it doesn't tend to have the same kind of bad > > impact on the call site. > > Very good to hear! Especially since I am not seeing how to move > ->rcu_read_unlock_special to a per-CPU variable given that rcu_boost() > needs cross-task access to it. There is probably some obvious trick, > but I will start with just __rcu_read_lock() for now.
And one obvious trick is a per-CPU pointer to the task-structure variable. But __rcu_read_lock() first.
Thanx, Paul
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