On Wed, May 29, 2019 at 01:54:06PM +1000, Tobin C. Harding wrote: > On Tue, May 21, 2019 at 02:05:38AM +0000, Roman Gushchin wrote: > > On Tue, May 21, 2019 at 11:31:18AM +1000, Tobin C. Harding wrote: > > > On Tue, May 21, 2019 at 12:57:47AM +0000, Roman Gushchin wrote: > > > > On Mon, May 20, 2019 at 03:40:17PM +1000, Tobin C. Harding wrote: > > > > > In an attempt to make the SMO patchset as non-invasive as possible add a > > > > > config option CONFIG_DCACHE_SMO (under "Memory Management options") for > > > > > enabling SMO for the DCACHE. Whithout this option dcache constructor is > > > > > used but no other code is built in, with this option enabled slab > > > > > mobility is enabled and the isolate/migrate functions are built in. > > > > > > > > > > Add CONFIG_DCACHE_SMO to guard the partial shrinking of the dcache via > > > > > Slab Movable Objects infrastructure. > > > > > > > > Hm, isn't it better to make it a static branch? Or basically anything > > > > that allows switching on the fly? > > > > > > If that is wanted, turning SMO on and off per cache, we can probably do > > > this in the SMO code in SLUB. > > > > Not necessarily per cache, but without recompiling the kernel. > > > > > > > It seems that the cost of just building it in shouldn't be that high. > > > > And the question if the defragmentation worth the trouble is so much > > > > easier to answer if it's possible to turn it on and off without rebooting. > > > > > > If the question is 'is defragmentation worth the trouble for the > > > dcache', I'm not sure having SMO turned off helps answer that question. > > > If one doesn't shrink the dentry cache there should be very little > > > overhead in having SMO enabled. So if one wants to explore this > > > question then they can turn on the config option. Please correct me if > > > I'm wrong. > > > > The problem with a config option is that it's hard to switch over. > > > > So just to test your changes in production a new kernel should be built, > > tested and rolled out to a representative set of machines (which can be > > measured in thousands of machines). Then if results are questionable, > > it should be rolled back. > > > > What you're actually guarding is the kmem_cache_setup_mobility() call, > > which can be perfectly avoided using a boot option, for example. Turning > > it on and off completely dynamic isn't that hard too. > > Hi Roman, > > I've added a boot parameter to SLUB so that admins can enable/disable > SMO at boot time system wide. Then for each object that implements SMO > (currently XArray and dcache) I've also added a boot parameter to > enable/disable SMO for that cache specifically (these depend on SMO > being enabled system wide). > > All three boot parameters default to 'off', I've added a config option > to default each to 'on'. > > I've got a little more testing to do on another part of the set then the > PATCH version is coming at you :) > > This is more a courtesy email than a request for comment, but please > feel free to shout if you don't like the method outlined above. > > Fully dynamic config is not currently possible because currently the SMO > implementation does not support disabling mobility for a cache once it > is turned on, a bit of extra logic would need to be added and some state > stored - I'm not sure it warrants it ATM but that can be easily added > later if wanted. Maybe Christoph will give his opinion on this. Perfect! Thanks.
