| Message ID | 20250319221634.71128-1-inwardvessel@gmail.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | cgroup: separate rstat trees | expand |
Please disregard this thread. It was sent with an incorrect version. A revised series labeled "v3" will be sent shortly. On 3/19/25 3:16 PM, JP Kobryn wrote: > The current design of rstat takes the approach that if one subsystem is to > be flushed, all other subsystems with pending updates should also be > flushed. A flush may be initiated by reading specific stats (like cpu.stat) > and other subsystems will be flushed alongside. The complexity of flushing > some subsystems has grown to the extent that the overhead of side flushes > is unnecessarily delaying the fetching of desired stats. > > One big area where the issue comes up is system telemetry, where programs > periodically sample cpu stats while the memory controller is enabled. It > would be a benefit for programs sampling cpu.stat if the overhead of having > to flush memory (and also io) stats was eliminated. It would save cpu > cycles for existing stat reader programs and improve scalability in terms > of sampling frequency and host volume. > > This series changes the approach of "flush all subsystems" to "flush only > the requested subsystem". The core design change is moving from a unified > model where rstat trees are shared by subsystems to having separate trees > for each subsystem. On a per-cpu basis, there will be separate trees for > each enabled subsystem if it implements css_rstat_flush and one tree for > the base stats. In order to do this, the rstat list pointers were moved off > of the cgroup and onto the css. In the transition, these pointer types were > changed to cgroup_subsys_state. Finally the API for updated/flush was > changed to accept a reference to a css instead of a cgroup. This allows for > a specific subsystem to be associated with a given update or flush. The > result is that rstat trees will now be made up of css nodes, and a given > tree will only contain nodes associated with a specific subsystem. > > Since separate trees will now be in use, the locking scheme was adjusted. > The global locks were split up in such a way that there are separate locks > for the base stats (cgroup::self) and also for each subsystem (memory, io, > etc). This allows different subsystems (and base stats) to use rstat in > parallel with no contention. > > Breaking up the unified tree into separate trees eliminates the overhead > and scalability issue explained in the first section, but comes at the > expense of additional memory. In an effort to minimize this overhead, new > rstat structs are introduced and a conditional allocation is performed. > The cgroup_rstat_cpu which originally contained the rstat list pointers and > the base stat entities was renamed cgroup_rstat_base_cpu. It is only > allocated when the associated css is cgroup::self. As for non-self css's, a > new compact struct was added that only contains the rstat list pointers. > During initialization, when the given css is associated with an actual > subsystem (not cgroup::self), this compact struct is allocated. With this > conditional allocation, the change in memory overhead on a per-cpu basis > before/after is shown below. > > memory overhead before: > sizeof(struct cgroup_rstat_cpu) =~ 144 bytes /* can vary based on config */ > > nr_cgroups * sizeof(struct cgroup_rstat_cpu) > nr_cgroups * 144 bytes > > memory overhead after: > sizeof(struct cgroup_rstat_cpu) == 16 bytes > sizeof(struct cgroup_rstat_base_cpu) =~ 144 bytes > > nr_cgroups * ( > sizeof(struct cgroup_rstat_base_cpu) + > sizeof(struct cgroup_rstat_cpu) * nr_rstat_controllers > ) > > nr_cgroups * (144 + 16 * nr_rstat_controllers) > > ... where nr_rstat_controllers is the number of enabled cgroup controllers > that implement css_rstat_flush(). On a host where both memory and io are > enabled: > > nr_cgroups * (144 + 16 * 2) > nr_cgroups * 176 bytes > > This leaves us with an increase in memory overhead of: > 32 bytes per cgroup per cpu > > Validation was performed by reading some *.stat files of a target parent > cgroup while the system was under different workloads. A test program was > made to loop 1M times, reading the files cgroup.stat, cpu.stat, io.stat, > memory.stat of the parent cgroup each iteration. Using a non-patched kernel > as control and this series as experimental, the findings show perf gains > when reading stats with this series. > > The first experiment consisted of a parent cgroup with memory.swap.max=0 > and memory.max=1G. On a 52-cpu machine, 26 child cgroups were created and > within each child cgroup a process was spawned to frequently update the > memory cgroup stats by creating and then reading a file of size 1T > (encouraging reclaim). The test program was run alongside these 26 tasks in > parallel. The results showed time and perf gains for the reader test > program. > > test program elapsed time > control: > real 1m13.663s > user 0m0.948s > sys 1m12.356s > > experiment: > real 0m42.498s > user 0m0.764s > sys 0m41.546s > > test program perf > control: > 31.75% mem_cgroup_css_rstat_flush > 5.49% __blkcg_rstat_flush > 0.10% cpu_stat_show > 0.05% cgroup_base_stat_cputime_show > > experiment: > 8.60% mem_cgroup_css_rstat_flush > 0.15% blkcg_print_stat > 0.12% cgroup_base_stat_cputime_show > > It's worth noting that memcg uses heuristics to optimize flushing. > Depending on the state of updated stats at a given time, a memcg flush may > be considered unnecessary and skipped as a result. This opportunity to skip > a flush is bypassed when memcg is flushed as a consequence of sharing the > tree with another controller. > > A second experiment was setup on the same host using a parent cgroup with > two child cgroups. The same swap and memory max were used as in the > previous experiment. In the two child cgroups, kernel builds were done in > parallel, each using "-j 20". The perf comparison is shown below. > > test program elapsed time > control: > real 1m22.809s > user 0m1.142s > sys 1m21.138s > > experiment: > real 0m42.504s > user 0m1.000s > sys 0m41.220s > > test program perf > control: > 37.16% mem_cgroup_css_rstat_flush > 3.68% __blkcg_rstat_flush > 0.09% cpu_stat_show > 0.06% cgroup_base_stat_cputime_show > > experiment: > 2.02% mem_cgroup_css_rstat_flush > 0.20% blkcg_print_stat > 0.14% cpu_stat_show > 0.08% cgroup_base_stat_cputime_show > > The final experiment differs from the previous two in that it measures > performance from the stat updater perspective. A kernel build was run in a > child node with -j 20 on the same host and cgroup setup. A baseline was > established by having the build run while no stats were read. The builds > were then repeated while stats were constantly being read. In all cases, > perf appeared similar in cycles spent on cgroup_rstat_updated() > (insignificant compared to the other recorded events). As for the elapsed > build times, the results of the different scenarios are shown below, > indicating no significant drawbacks of the split tree approach. > > control with no readers > real 5m12.307s > user 84m52.037s > sys 3m54.000s > > control with constant readers of {memory,io,cpu,cgroup}.stat > real 5m13.209s > user 84m47.949s > sys 4m9.260s > > experiment with no readers > real 5m11.961s > user 84m41.750s > sys 3m54.058s > > experiment with constant readers of {memory,io,cpu,cgroup}.stat > real 5m12.626s > user 85m0.323s > sys 3m56.167s > > changelog > v3: > new bpf kfunc api for updated/flush > rename cgroup_rstat_{updated,flush} and related to "css_rstat_*" > check for ss->css_rstat_flush existence where applicable > rename locks for base stats > move subsystem locks to cgroup_subsys struct > change cgroup_rstat_boot() to ss_rstat_init(ss) and init locks within > change lock helpers to accept css and perform lock selection within > fix comments that had outdated lock names > add open css_is_cgroup() helper > rename rstatc to rstatbc to reflect base stats in use > rename cgroup_dfl_root_rstat_cpu to root_self_rstat_cpu > add comments in early init code to explain deferred allocation > misc formatting fixes > > v2: > drop the patch creating a new cgroup_rstat struct and related code > drop bpf-specific patches. instead just use cgroup::self in bpf progs > drop the cpu lock patches. instead select cpu lock in updated_list func > relocate the cgroup_rstat_init() call to inside css_create() > relocate the cgroup_rstat_exit() cleanup from apply_control_enable() > to css_free_rwork_fn() > v1: > https://lore.kernel.org/all/20250218031448.46951-1-inwardvessel@gmail.com/ > > JP Kobryn (4): > cgroup: separate rstat api for bpf programs > cgroup: use separate rstat trees for each subsystem > cgroup: use subsystem-specific rstat locks to avoid contention > cgroup: split up cgroup_rstat_cpu into base stat and non base stat > versions > > block/blk-cgroup.c | 6 +- > include/linux/cgroup-defs.h | 80 ++-- > include/linux/cgroup.h | 16 +- > include/trace/events/cgroup.h | 10 +- > kernel/cgroup/cgroup-internal.h | 6 +- > kernel/cgroup/cgroup.c | 69 +-- > kernel/cgroup/rstat.c | 412 +++++++++++------- > mm/memcontrol.c | 4 +- > .../selftests/bpf/progs/btf_type_tag_percpu.c | 5 +- > .../bpf/progs/cgroup_hierarchical_stats.c | 8 +- > 10 files changed, 363 insertions(+), 253 deletions(-) >
The current design of rstat takes the approach that if one subsystem is to be flushed, all other subsystems with pending updates should also be flushed. A flush may be initiated by reading specific stats (like cpu.stat) and other subsystems will be flushed alongside. The complexity of flushing some subsystems has grown to the extent that the overhead of side flushes is unnecessarily delaying the fetching of desired stats. One big area where the issue comes up is system telemetry, where programs periodically sample cpu stats while the memory controller is enabled. It would be a benefit for programs sampling cpu.stat if the overhead of having to flush memory (and also io) stats was eliminated. It would save cpu cycles for existing stat reader programs and improve scalability in terms of sampling frequency and host volume. This series changes the approach of "flush all subsystems" to "flush only the requested subsystem". The core design change is moving from a unified model where rstat trees are shared by subsystems to having separate trees for each subsystem. On a per-cpu basis, there will be separate trees for each enabled subsystem if it implements css_rstat_flush and one tree for the base stats. In order to do this, the rstat list pointers were moved off of the cgroup and onto the css. In the transition, these pointer types were changed to cgroup_subsys_state. Finally the API for updated/flush was changed to accept a reference to a css instead of a cgroup. This allows for a specific subsystem to be associated with a given update or flush. The result is that rstat trees will now be made up of css nodes, and a given tree will only contain nodes associated with a specific subsystem. Since separate trees will now be in use, the locking scheme was adjusted. The global locks were split up in such a way that there are separate locks for the base stats (cgroup::self) and also for each subsystem (memory, io, etc). This allows different subsystems (and base stats) to use rstat in parallel with no contention. Breaking up the unified tree into separate trees eliminates the overhead and scalability issue explained in the first section, but comes at the expense of additional memory. In an effort to minimize this overhead, new rstat structs are introduced and a conditional allocation is performed. The cgroup_rstat_cpu which originally contained the rstat list pointers and the base stat entities was renamed cgroup_rstat_base_cpu. It is only allocated when the associated css is cgroup::self. As for non-self css's, a new compact struct was added that only contains the rstat list pointers. During initialization, when the given css is associated with an actual subsystem (not cgroup::self), this compact struct is allocated. With this conditional allocation, the change in memory overhead on a per-cpu basis before/after is shown below. memory overhead before: sizeof(struct cgroup_rstat_cpu) =~ 144 bytes /* can vary based on config */ nr_cgroups * sizeof(struct cgroup_rstat_cpu) nr_cgroups * 144 bytes memory overhead after: sizeof(struct cgroup_rstat_cpu) == 16 bytes sizeof(struct cgroup_rstat_base_cpu) =~ 144 bytes nr_cgroups * ( sizeof(struct cgroup_rstat_base_cpu) + sizeof(struct cgroup_rstat_cpu) * nr_rstat_controllers ) nr_cgroups * (144 + 16 * nr_rstat_controllers) ... where nr_rstat_controllers is the number of enabled cgroup controllers that implement css_rstat_flush(). On a host where both memory and io are enabled: nr_cgroups * (144 + 16 * 2) nr_cgroups * 176 bytes This leaves us with an increase in memory overhead of: 32 bytes per cgroup per cpu Validation was performed by reading some *.stat files of a target parent cgroup while the system was under different workloads. A test program was made to loop 1M times, reading the files cgroup.stat, cpu.stat, io.stat, memory.stat of the parent cgroup each iteration. Using a non-patched kernel as control and this series as experimental, the findings show perf gains when reading stats with this series. The first experiment consisted of a parent cgroup with memory.swap.max=0 and memory.max=1G. On a 52-cpu machine, 26 child cgroups were created and within each child cgroup a process was spawned to frequently update the memory cgroup stats by creating and then reading a file of size 1T (encouraging reclaim). The test program was run alongside these 26 tasks in parallel. The results showed time and perf gains for the reader test program. test program elapsed time control: real 1m13.663s user 0m0.948s sys 1m12.356s experiment: real 0m42.498s user 0m0.764s sys 0m41.546s test program perf control: 31.75% mem_cgroup_css_rstat_flush 5.49% __blkcg_rstat_flush 0.10% cpu_stat_show 0.05% cgroup_base_stat_cputime_show experiment: 8.60% mem_cgroup_css_rstat_flush 0.15% blkcg_print_stat 0.12% cgroup_base_stat_cputime_show It's worth noting that memcg uses heuristics to optimize flushing. Depending on the state of updated stats at a given time, a memcg flush may be considered unnecessary and skipped as a result. This opportunity to skip a flush is bypassed when memcg is flushed as a consequence of sharing the tree with another controller. A second experiment was setup on the same host using a parent cgroup with two child cgroups. The same swap and memory max were used as in the previous experiment. In the two child cgroups, kernel builds were done in parallel, each using "-j 20". The perf comparison is shown below. test program elapsed time control: real 1m22.809s user 0m1.142s sys 1m21.138s experiment: real 0m42.504s user 0m1.000s sys 0m41.220s test program perf control: 37.16% mem_cgroup_css_rstat_flush 3.68% __blkcg_rstat_flush 0.09% cpu_stat_show 0.06% cgroup_base_stat_cputime_show experiment: 2.02% mem_cgroup_css_rstat_flush 0.20% blkcg_print_stat 0.14% cpu_stat_show 0.08% cgroup_base_stat_cputime_show The final experiment differs from the previous two in that it measures performance from the stat updater perspective. A kernel build was run in a child node with -j 20 on the same host and cgroup setup. A baseline was established by having the build run while no stats were read. The builds were then repeated while stats were constantly being read. In all cases, perf appeared similar in cycles spent on cgroup_rstat_updated() (insignificant compared to the other recorded events). As for the elapsed build times, the results of the different scenarios are shown below, indicating no significant drawbacks of the split tree approach. control with no readers real 5m12.307s user 84m52.037s sys 3m54.000s control with constant readers of {memory,io,cpu,cgroup}.stat real 5m13.209s user 84m47.949s sys 4m9.260s experiment with no readers real 5m11.961s user 84m41.750s sys 3m54.058s experiment with constant readers of {memory,io,cpu,cgroup}.stat real 5m12.626s user 85m0.323s sys 3m56.167s changelog v3: new bpf kfunc api for updated/flush rename cgroup_rstat_{updated,flush} and related to "css_rstat_*" check for ss->css_rstat_flush existence where applicable rename locks for base stats move subsystem locks to cgroup_subsys struct change cgroup_rstat_boot() to ss_rstat_init(ss) and init locks within change lock helpers to accept css and perform lock selection within fix comments that had outdated lock names add open css_is_cgroup() helper rename rstatc to rstatbc to reflect base stats in use rename cgroup_dfl_root_rstat_cpu to root_self_rstat_cpu add comments in early init code to explain deferred allocation misc formatting fixes v2: drop the patch creating a new cgroup_rstat struct and related code drop bpf-specific patches. instead just use cgroup::self in bpf progs drop the cpu lock patches. instead select cpu lock in updated_list func relocate the cgroup_rstat_init() call to inside css_create() relocate the cgroup_rstat_exit() cleanup from apply_control_enable() to css_free_rwork_fn() v1: https://lore.kernel.org/all/20250218031448.46951-1-inwardvessel@gmail.com/ JP Kobryn (4): cgroup: separate rstat api for bpf programs cgroup: use separate rstat trees for each subsystem cgroup: use subsystem-specific rstat locks to avoid contention cgroup: split up cgroup_rstat_cpu into base stat and non base stat versions block/blk-cgroup.c | 6 +- include/linux/cgroup-defs.h | 80 ++-- include/linux/cgroup.h | 16 +- include/trace/events/cgroup.h | 10 +- kernel/cgroup/cgroup-internal.h | 6 +- kernel/cgroup/cgroup.c | 69 +-- kernel/cgroup/rstat.c | 412 +++++++++++------- mm/memcontrol.c | 4 +- .../selftests/bpf/progs/btf_type_tag_percpu.c | 5 +- .../bpf/progs/cgroup_hierarchical_stats.c | 8 +- 10 files changed, 363 insertions(+), 253 deletions(-)