Hi folks, This series has been in the mm-unstable for several months. Are there any remaining concerns here otherwise can we please put this in the mm-stable branch to be merged in the next Linux release? On Wed, Jan 03, 2024 at 08:48:35AM GMT, Dan Schatzberg wrote: > Changes since V5: > * Made the scan_control behavior limited to proactive reclaim explicitly > * created sc_swappiness helper to reduce chance of mis-use > > Changes since V4: > * Fixed some initialization bugs by reverting back to a pointer for swappiness > * Added some more caveats to the behavior of swappiness in documentation > > Changes since V3: > * Added #define for MIN_SWAPPINESS and MAX_SWAPPINESS > * Added explicit calls to mem_cgroup_swappiness > > Changes since V2: > * No functional change > * Used int consistently rather than a pointer > > Changes since V1: > * Added documentation > > This patch proposes augmenting the memory.reclaim interface with a > swappiness=<val> argument that overrides the swappiness value for that instance > of proactive reclaim. > > Userspace proactive reclaimers use the memory.reclaim interface to trigger > reclaim. The memory.reclaim interface does not allow for any way to effect the > balance of file vs anon during proactive reclaim. The only approach is to adjust > the vm.swappiness setting. However, there are a few reasons we look to control > the balance of file vs anon during proactive reclaim, separately from reactive > reclaim: > > * Swapout should be limited to manage SSD write endurance. In near-OOM > situations we are fine with lots of swap-out to avoid OOMs. As these are > typically rare events, they have relatively little impact on write endurance. > However, proactive reclaim runs continuously and so its impact on SSD write > endurance is more significant. Therefore it is desireable to control swap-out > for proactive reclaim separately from reactive reclaim > > * Some userspace OOM killers like systemd-oomd[1] support OOM killing on swap > exhaustion. This makes sense if the swap exhaustion is triggered due to > reactive reclaim but less so if it is triggered due to proactive reclaim (e.g. > one could see OOMs when free memory is ample but anon is just particularly > cold). Therefore, it's desireable to have proactive reclaim reduce or stop > swap-out before the threshold at which OOM killing occurs. > > In the case of Meta's Senpai proactive reclaimer, we adjust vm.swappiness before > writes to memory.reclaim[2]. This has been in production for nearly two years > and has addressed our needs to control proactive vs reactive reclaim behavior > but is still not ideal for a number of reasons: > > * vm.swappiness is a global setting, adjusting it can race/interfere with other > system administration that wishes to control vm.swappiness. In our case, we > need to disable Senpai before adjusting vm.swappiness. > > * vm.swappiness is stateful - so a crash or restart of Senpai can leave a > misconfigured setting. This requires some additional management to record the > "desired" setting and ensure Senpai always adjusts to it. > > With this patch, we avoid these downsides of adjusting vm.swappiness globally. > > Previously, this exact interface addition was proposed by Yosry[3]. In response, > Roman proposed instead an interface to specify precise file/anon/slab reclaim > amounts[4]. More recently Huan also proposed this as well[5] and others > similarly questioned if this was the proper interface. > > Previous proposals sought to use this to allow proactive reclaimers to > effectively perform a custom reclaim algorithm by issuing proactive reclaim with > different settings to control file vs anon reclaim (e.g. to only reclaim anon > from some applications). Responses argued that adjusting swappiness is a poor > interface for custom reclaim. > > In contrast, I argue in favor of a swappiness setting not as a way to implement > custom reclaim algorithms but rather to bias the balance of anon vs file due to > differences of proactive vs reactive reclaim. In this context, swappiness is the > existing interface for controlling this balance and this patch simply allows for > it to be configured differently for proactive vs reactive reclaim. > > Specifying explicit amounts of anon vs file pages to reclaim feels inappropriate > for this prupose. Proactive reclaimers are un-aware of the relative age of file > vs anon for a cgroup which makes it difficult to manage proactive reclaim of > different memory pools. A proactive reclaimer would need some amount of anon > reclaim attempts separate from the amount of file reclaim attempts which seems > brittle given that it's difficult to observe the impact. > > [1]https://www.freedesktop.org/software/systemd/man/latest/systemd-oomd.service.html > [2]https://github.com/facebookincubator/oomd/blob/main/src/oomd/plugins/Senpai.cpp#L585-L598 > [3]https://lore.kernel.org/linux-mm/CAJD7tkbDpyoODveCsnaqBBMZEkDvshXJmNdbk51yKSNgD7aGdg@xxxxxxxxxxxxxx/ > [4]https://lore.kernel.org/linux-mm/YoPHtHXzpK51F%2F1Z@carbon/ > [5]https://lore.kernel.org/lkml/20231108065818.19932-1-link@xxxxxxxx/ > > Dan Schatzberg (2): > mm: add defines for min/max swappiness > mm: add swapiness= arg to memory.reclaim > > Documentation/admin-guide/cgroup-v2.rst | 18 +++++--- > include/linux/swap.h | 5 ++- > mm/memcontrol.c | 58 ++++++++++++++++++++----- > mm/vmscan.c | 39 ++++++++++++----- > 4 files changed, 90 insertions(+), 30 deletions(-) > > -- > 2.39.3 >
