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. Of course, it's up to you, it's just probably easier to find new users of a new feature, when it's easy to test it. Thanks!
