On Mon, May 04, 2020 at 07:48:22PM +0200, Uladzislau Rezki wrote: > On Mon, May 04, 2020 at 08:24:37AM -0700, Paul E. McKenney wrote: > > On Mon, May 04, 2020 at 02:43:23PM +0200, Uladzislau Rezki wrote: > > > On Fri, May 01, 2020 at 02:27:49PM -0700, Paul E. McKenney wrote: > > > > On Tue, Apr 28, 2020 at 10:58:48PM +0200, Uladzislau Rezki (Sony) wrote: > > > > > Cache some extra objects per-CPU. During reclaim process > > > > > some pages are cached instead of releasing by linking them > > > > > into the list. Such approach provides O(1) access time to > > > > > the cache. > > > > > > > > > > That reduces number of requests to the page allocator, also > > > > > that makes it more helpful if a low memory condition occurs. > > > > > > > > > > A parameter reflecting the minimum allowed pages to be > > > > > cached per one CPU is propagated via sysfs, it is read > > > > > only, the name is "rcu_min_cached_objs". > > > > > > > > > > Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx> > > > > > --- > > > > > kernel/rcu/tree.c | 64 ++++++++++++++++++++++++++++++++++++++++++++--- > > > > > 1 file changed, 60 insertions(+), 4 deletions(-) > > > > > > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > > > > index 89e9ca3f4e3e..d8975819b1c9 100644 > > > > > --- a/kernel/rcu/tree.c > > > > > +++ b/kernel/rcu/tree.c > > > > > @@ -178,6 +178,14 @@ module_param(gp_init_delay, int, 0444); > > > > > static int gp_cleanup_delay; > > > > > module_param(gp_cleanup_delay, int, 0444); > > > > > > > > > > +/* > > > > > + * This rcu parameter is read-only, but can be write also. > > > > > > > > You mean that although the parameter is read-only, you see no reason > > > > why it could not be converted to writeable? > > > > > > > I added just a note. If it is writable, then we can change the size of the > > > per-CPU cache dynamically, i.e. "echo 5 > /sys/.../rcu_min_cached_objs" > > > would cache 5 pages. But i do not have a strong opinion if it should be > > > writable. > > > > > > > If it was writeable, and a given CPU had the maximum numbr of cached > > > > objects, the rcu_min_cached_objs value was decreased, but that CPU never > > > > saw another kfree_rcu(), would the number of cached objects change? > > > > > > > No. It works the way: unqueue the page from cache in the kfree_rcu(), > > > whereas "rcu work" will put it back if number of objects < rcu_min_cached_objs, > > > if >= will free the page. > > > > Just to make sure I understand... If someone writes a smaller number to > > the sysfs variable, the per-CPU caches will be decreased at that point, > > immediately during that sysfs write? Or are you saying something else? > > > This patch defines it as read-only. It defines the minimum threshold that > controls number of elements in the per-CPU cache. If we decide to make it > write also, then we will have full of freedom how to define its behavior, > i.e. it is not defined because it is read only. And runtime-read-only sounds like an excellent state for it. > > > > Presumably the list can also be accessed without holding this lock, > > > > because otherwise we shouldn't need llist... > > > > > > > Hm... We increase the number of elements in cache, therefore it is not > > > lockless. From the other hand i used llist_head to maintain the cache > > > because it is single linked list, we do not need "*prev" link. Also > > > we do not need to init the list. > > > > > > But i can change it to list_head. Please let me know if i need :) > > > > Hmmm... Maybe it is time for a non-atomic singly linked list? In the RCU > > callback processing, the operations were open-coded, but they have been > > pushed into include/linux/rcu_segcblist.h and kernel/rcu/rcu_segcblist.*. > > > > Maybe some non-atomic/protected/whatever macros in the llist.h file? > > Or maybe just open-code the singly linked list? (Probably not the > > best choice, though.) Add comments stating that the atomic properties > > of the llist functions aren't neded? Something else? > > > In order to keep it simple i can replace llist_head by the list_head? Fine by me! > > The comments would be a good start. Just to take pity on people seeing > > the potential for concurrency and wondering how the concurrent accesses > > actually happen. ;-) > > > Sounds like you are kidding me :) "Only those who have gone too far can possibly tell you how far you can go!" ;-) Thanx, Paul