Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Wed, Apr 19, 2017 at 05:40:40PM +0200, Peter Zijlstra wrote: > On Wed, Apr 19, 2017 at 08:08:09AM -0700, Paul E. McKenney wrote: > > And even that would not be completely sufficient. After all, the state > > in the leaf rcu_node structure will be out of date during grace-period > >

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Wed, Apr 19, 2017 at 05:40:40PM +0200, Peter Zijlstra wrote: > On Wed, Apr 19, 2017 at 08:08:09AM -0700, Paul E. McKenney wrote: > > And even that would not be completely sufficient. After all, the state > > in the leaf rcu_node structure will be out of date during grace-period > >

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Wed, Apr 19, 2017 at 08:08:09AM -0700, Paul E. McKenney wrote: > And even that would not be completely sufficient. After all, the state > in the leaf rcu_node structure will be out of date during grace-period > initialization and cleanup. So to -completely- synchronize state for > the

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Wed, Apr 19, 2017 at 08:08:09AM -0700, Paul E. McKenney wrote: > And even that would not be completely sufficient. After all, the state > in the leaf rcu_node structure will be out of date during grace-period > initialization and cleanup. So to -completely- synchronize state for > the

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Wed, Apr 19, 2017 at 03:48:35PM +0200, Peter Zijlstra wrote: > On Wed, Apr 19, 2017 at 03:22:26PM +0200, Peter Zijlstra wrote: > > On Thu, Apr 13, 2017 at 11:42:32AM -0700, Paul E. McKenney wrote: > > > > > I believe that you are missing the fact that RCU grace-period > > > initialization and

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Wed, Apr 19, 2017 at 03:48:35PM +0200, Peter Zijlstra wrote: > On Wed, Apr 19, 2017 at 03:22:26PM +0200, Peter Zijlstra wrote: > > On Thu, Apr 13, 2017 at 11:42:32AM -0700, Paul E. McKenney wrote: > > > > > I believe that you are missing the fact that RCU grace-period > > > initialization and

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Wed, Apr 19, 2017 at 03:22:26PM +0200, Peter Zijlstra wrote: > On Thu, Apr 13, 2017 at 11:42:32AM -0700, Paul E. McKenney wrote: > > > I believe that you are missing the fact that RCU grace-period > > initialization and cleanup walks through the rcu_node tree breadth > > first, using

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Wed, Apr 19, 2017 at 03:22:26PM +0200, Peter Zijlstra wrote: > On Thu, Apr 13, 2017 at 11:42:32AM -0700, Paul E. McKenney wrote: > > > I believe that you are missing the fact that RCU grace-period > > initialization and cleanup walks through the rcu_node tree breadth > > first, using

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Wed, Apr 19, 2017 at 03:22:26PM +0200, Peter Zijlstra wrote: > On Thu, Apr 13, 2017 at 11:42:32AM -0700, Paul E. McKenney wrote: > > > I believe that you are missing the fact that RCU grace-period > > initialization and cleanup walks through the rcu_node tree breadth > > first, using

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Wed, Apr 19, 2017 at 03:22:26PM +0200, Peter Zijlstra wrote: > On Thu, Apr 13, 2017 at 11:42:32AM -0700, Paul E. McKenney wrote: > > > I believe that you are missing the fact that RCU grace-period > > initialization and cleanup walks through the rcu_node tree breadth > > first, using

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Thu, Apr 13, 2017 at 11:42:32AM -0700, Paul E. McKenney wrote: > I believe that you are missing the fact that RCU grace-period > initialization and cleanup walks through the rcu_node tree breadth > first, using rcu_for_each_node_breadth_first(). Indeed. That is the part I completely missed.

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Thu, Apr 13, 2017 at 11:42:32AM -0700, Paul E. McKenney wrote: > I believe that you are missing the fact that RCU grace-period > initialization and cleanup walks through the rcu_node tree breadth > first, using rcu_for_each_node_breadth_first(). Indeed. That is the part I completely missed.

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Thu, Apr 13, 2017 at 08:29:39PM +0200, Peter Zijlstra wrote: > On Thu, Apr 13, 2017 at 10:31:00AM -0700, Paul E. McKenney wrote: > > On Thu, Apr 13, 2017 at 07:04:34PM +0200, Peter Zijlstra wrote: > > > > And I have vague memories of it actually causing lock contention, but > > > I've

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Thu, Apr 13, 2017 at 08:29:39PM +0200, Peter Zijlstra wrote: > On Thu, Apr 13, 2017 at 10:31:00AM -0700, Paul E. McKenney wrote: > > On Thu, Apr 13, 2017 at 07:04:34PM +0200, Peter Zijlstra wrote: > > > > And I have vague memories of it actually causing lock contention, but > > > I've

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Thu, Apr 13, 2017 at 08:23:09PM +0200, Peter Zijlstra wrote: > On Thu, Apr 13, 2017 at 11:19:26AM -0700, Paul E. McKenney wrote: > > > First get me some system-level data showing that the current layout is > > causing a real problem. RCU's fastpath code doesn't come anywhere near > > the

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Thu, Apr 13, 2017 at 08:23:09PM +0200, Peter Zijlstra wrote: > On Thu, Apr 13, 2017 at 11:19:26AM -0700, Paul E. McKenney wrote: > > > First get me some system-level data showing that the current layout is > > causing a real problem. RCU's fastpath code doesn't come anywhere near > > the

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Thu, Apr 13, 2017 at 10:31:00AM -0700, Paul E. McKenney wrote: > On Thu, Apr 13, 2017 at 07:04:34PM +0200, Peter Zijlstra wrote: > > And I have vague memories of it actually causing lock contention, but > > I've forgotten how that worked. > > That is a new one on me. I can easily see how not

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Thu, Apr 13, 2017 at 10:31:00AM -0700, Paul E. McKenney wrote: > On Thu, Apr 13, 2017 at 07:04:34PM +0200, Peter Zijlstra wrote: > > And I have vague memories of it actually causing lock contention, but > > I've forgotten how that worked. > > That is a new one on me. I can easily see how not

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Thu, Apr 13, 2017 at 11:19:26AM -0700, Paul E. McKenney wrote: > First get me some system-level data showing that the current layout is > causing a real problem. RCU's fastpath code doesn't come anywhere near > the rcu_node tree, so in the absence of such data, I of course remain > quite

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Thu, Apr 13, 2017 at 11:19:26AM -0700, Paul E. McKenney wrote: > First get me some system-level data showing that the current layout is > causing a real problem. RCU's fastpath code doesn't come anywhere near > the rcu_node tree, so in the absence of such data, I of course remain > quite

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Thu, Apr 13, 2017 at 07:46:31PM +0200, Peter Zijlstra wrote: > On Thu, Apr 13, 2017 at 10:31:00AM -0700, Paul E. McKenney wrote: > > On Thu, Apr 13, 2017 at 07:04:34PM +0200, Peter Zijlstra wrote: > > > On Thu, Apr 13, 2017 at 09:55:16AM -0700, Paul E. McKenney wrote: > > > > > > To avoid

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Thu, Apr 13, 2017 at 07:46:31PM +0200, Peter Zijlstra wrote: > On Thu, Apr 13, 2017 at 10:31:00AM -0700, Paul E. McKenney wrote: > > On Thu, Apr 13, 2017 at 07:04:34PM +0200, Peter Zijlstra wrote: > > > On Thu, Apr 13, 2017 at 09:55:16AM -0700, Paul E. McKenney wrote: > > > > > > To avoid

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Thu, Apr 13, 2017 at 10:31:00AM -0700, Paul E. McKenney wrote: > On Thu, Apr 13, 2017 at 07:04:34PM +0200, Peter Zijlstra wrote: > > On Thu, Apr 13, 2017 at 09:55:16AM -0700, Paul E. McKenney wrote: > > > > > To avoid people tuning huge machines having to wait for me to give > > > > > them an

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Thu, Apr 13, 2017 at 10:31:00AM -0700, Paul E. McKenney wrote: > On Thu, Apr 13, 2017 at 07:04:34PM +0200, Peter Zijlstra wrote: > > On Thu, Apr 13, 2017 at 09:55:16AM -0700, Paul E. McKenney wrote: > > > > > To avoid people tuning huge machines having to wait for me to give > > > > > them an

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Thu, Apr 13, 2017 at 07:04:34PM +0200, Peter Zijlstra wrote: > On Thu, Apr 13, 2017 at 09:55:16AM -0700, Paul E. McKenney wrote: > > > > To avoid people tuning huge machines having to wait for me to give > > > > them an answer as to why they are suffering lock contention after > > > > cranking

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Thu, Apr 13, 2017 at 07:04:34PM +0200, Peter Zijlstra wrote: > On Thu, Apr 13, 2017 at 09:55:16AM -0700, Paul E. McKenney wrote: > > > > To avoid people tuning huge machines having to wait for me to give > > > > them an answer as to why they are suffering lock contention after > > > > cranking

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Thu, Apr 13, 2017 at 09:55:16AM -0700, Paul E. McKenney wrote: > > > To avoid people tuning huge machines having to wait for me to give > > > them an answer as to why they are suffering lock contention after > > > cranking up the value of RCU_FANOUT_LEAF. So is there a good reason to increase

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Thu, Apr 13, 2017 at 09:55:16AM -0700, Paul E. McKenney wrote: > > > To avoid people tuning huge machines having to wait for me to give > > > them an answer as to why they are suffering lock contention after > > > cranking up the value of RCU_FANOUT_LEAF. So is there a good reason to increase

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Thu, Apr 13, 2017 at 06:19:48PM +0200, Peter Zijlstra wrote: > On Thu, Apr 13, 2017 at 09:03:33AM -0700, Paul E. McKenney wrote: > > On Thu, Apr 13, 2017 at 11:15:35AM +0200, Peter Zijlstra wrote: > > > On Wed, Apr 12, 2017 at 09:55:40AM -0700, Paul E. McKenney wrote: > > > > If you set

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Thu, Apr 13, 2017 at 06:19:48PM +0200, Peter Zijlstra wrote: > On Thu, Apr 13, 2017 at 09:03:33AM -0700, Paul E. McKenney wrote: > > On Thu, Apr 13, 2017 at 11:15:35AM +0200, Peter Zijlstra wrote: > > > On Wed, Apr 12, 2017 at 09:55:40AM -0700, Paul E. McKenney wrote: > > > > If you set

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Thu, Apr 13, 2017 at 09:03:33AM -0700, Paul E. McKenney wrote: > On Thu, Apr 13, 2017 at 11:15:35AM +0200, Peter Zijlstra wrote: > > On Wed, Apr 12, 2017 at 09:55:40AM -0700, Paul E. McKenney wrote: > > > If you set RCU_FANOUT_LEAF too high, you can get lock contention > > > on the leaf

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Thu, Apr 13, 2017 at 09:03:33AM -0700, Paul E. McKenney wrote: > On Thu, Apr 13, 2017 at 11:15:35AM +0200, Peter Zijlstra wrote: > > On Wed, Apr 12, 2017 at 09:55:40AM -0700, Paul E. McKenney wrote: > > > If you set RCU_FANOUT_LEAF too high, you can get lock contention > > > on the leaf

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Thu, Apr 13, 2017 at 11:15:35AM +0200, Peter Zijlstra wrote: > On Wed, Apr 12, 2017 at 09:55:40AM -0700, Paul E. McKenney wrote: > > If you set RCU_FANOUT_LEAF too high, you can get lock contention > > on the leaf rcu_node, and you should boot with the skew_tick kernel > > parameter set in

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Thu, Apr 13, 2017 at 11:15:35AM +0200, Peter Zijlstra wrote: > On Wed, Apr 12, 2017 at 09:55:40AM -0700, Paul E. McKenney wrote: > > If you set RCU_FANOUT_LEAF too high, you can get lock contention > > on the leaf rcu_node, and you should boot with the skew_tick kernel > > parameter set in

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Wed, Apr 12, 2017 at 09:55:40AM -0700, Paul E. McKenney wrote: > If you set RCU_FANOUT_LEAF too high, you can get lock contention > on the leaf rcu_node, and you should boot with the skew_tick kernel > parameter set in order to avoid this lock contention. This commit > therefore upgrades the

Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

On Wed, Apr 12, 2017 at 09:55:40AM -0700, Paul E. McKenney wrote: > If you set RCU_FANOUT_LEAF too high, you can get lock contention > on the leaf rcu_node, and you should boot with the skew_tick kernel > parameter set in order to avoid this lock contention. This commit > therefore upgrades the

[PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

If you set RCU_FANOUT_LEAF too high, you can get lock contention on the leaf rcu_node, and you should boot with the skew_tick kernel parameter set in order to avoid this lock contention. This commit therefore upgrades the RCU_FANOUT_LEAF help text to explicitly state this. Signed-off-by: Paul E.

[PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

If you set RCU_FANOUT_LEAF too high, you can get lock contention on the leaf rcu_node, and you should boot with the skew_tick kernel parameter set in order to avoid this lock contention. This commit therefore upgrades the RCU_FANOUT_LEAF help text to explicitly state this. Signed-off-by: Paul E.