From: Hui Zhu <[email protected]> BPF programs can observe memory pressure on a cgroup (e.g. refault stats via bpf_mem_cgroup_page_state()), but cannot act on it: triggering reclaim on a chosen cgroup requires writing to memory.reclaim, which BPF cannot do. Add bpf_proactive_reclaim(), a sleepable kfunc which performs one proactive reclaim pass on a given memory cgroup, similar to a write to memory.reclaim but without retrying until the target is reached, so that when and how hard to reclaim is BPF policy rather than hard-coded thresholds.
Since some bpf program types may be invoked while holding fs locks, limit the kfunc to BPF_PROG_TYPE_SYSCALL only, to avoid deadlocking in filesystem shrinkers on the reclaim path. A SYSCALL program can invoke the kfunc directly, or asynchronously from its bpf_wq or task_work callbacks, which run in process context and keep the SYSCALL program type. The reclaim target of a single call is capped at MEMCG_CHARGE_BATCH, following the precedent of high_work_func(), the memory.high workqueue fallback, which bounds each reclaim request the same way and uses one work item per memcg. Note that only the reclaim target is capped: the actual scanning work and its duration are not bounded. Reclaiming more than one batch is left to the BPF program rather than enforced by the kfunc: with one call per bpf_wq callback and the same work item requeued for the next batch, the program can also stop submitting batches in between, e.g. once the target cgroup is dying. Signed-off-by: Hui Zhu <[email protected]> --- mm/bpf_memcontrol.c | 88 ++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 87 insertions(+), 1 deletion(-) diff --git a/mm/bpf_memcontrol.c b/mm/bpf_memcontrol.c index 716df49d7647..2817c6db9a5d 100644 --- a/mm/bpf_memcontrol.c +++ b/mm/bpf_memcontrol.c @@ -6,8 +6,11 @@ */ #include <linux/memcontrol.h> +#include <linux/swap.h> #include <linux/bpf.h> +#include "internal.h" + __bpf_kfunc_start_defs(); /** @@ -159,6 +162,73 @@ __bpf_kfunc void bpf_mem_cgroup_flush_stats(struct mem_cgroup *memcg) mem_cgroup_flush_stats(memcg); } +/** + * bpf_proactive_reclaim - proactively reclaim memory from a memory + * cgroup + * @memcg: the target memory cgroup to reclaim from + * @size: the amount of memory to reclaim, in bytes, clamped to + * MEMCG_CHARGE_BATCH (64 pages) + * @swappiness: the reclaim swappiness, in the range [0, 201] where 201 + * means anon-only reclaim; a negative value means the memcg's + * own swappiness is used + * + * Trigger one proactive reclaim pass on @memcg, similar to a write to + * memory.reclaim, but without retrying until @size is reached. + * + * Only the reclaim target is capped: @size is clamped to + * MEMCG_CHARGE_BATCH, following the precedent of high_work_func(), + * the memory.high workqueue fallback, which bounds each reclaim + * request the same way. The actual scanning work and its duration + * are not bounded. To reclaim more, call this kfunc repeatedly + * instead of passing a larger @size. + * + * The kfunc can be called directly from a BPF_PROG_TYPE_SYSCALL + * program, synchronously in the context of the thread running the + * program, or from the bpf_wq and task_work callbacks of a SYSCALL + * program, which run in process context and keep the SYSCALL program + * type. It is registered for BPF_PROG_TYPE_SYSCALL only, because + * generic sleepable programs may run with filesystem locks held or + * in NOFS/NOIO contexts, where the reclaim path could deadlock on + * those locks via filesystem shrinkers. + * + * For asynchronous reclaim of more than one batch, driving the + * reclaim from a bpf_wq callback is recommended: call this kfunc + * once per callback and requeue the same work item for the next + * batch, instead of looping inside the callback and monopolizing a + * workqueue worker, and give each target memcg its own work item, + * as high_work_func() does with one work item per memcg. Whether + * to submit the next batch is up to the BPF program, which can stop + * at any point, e.g. once the target cgroup is dying. + * + * Return: The amount of memory reclaimed, in bytes, or 0 if @size is + * smaller than a page, or (unsigned long)-1 if @swappiness is out of + * range. + */ +__bpf_kfunc unsigned long bpf_proactive_reclaim(struct mem_cgroup *memcg, + unsigned long size, + int swappiness) +{ + unsigned long nr_reclaimed; + unsigned long nr_pages; + + if (swappiness < -1 || swappiness > SWAPPINESS_ANON_ONLY) + return (unsigned long)-1; + + if (size < PAGE_SIZE) + return 0; + + nr_pages = min(size / PAGE_SIZE, (unsigned long)MEMCG_CHARGE_BATCH); + + nr_reclaimed = try_to_free_mem_cgroup_pages(memcg, nr_pages, + GFP_KERNEL, + MEMCG_RECLAIM_MAY_SWAP | + MEMCG_RECLAIM_PROACTIVE, + swappiness < 0 ? NULL : + &swappiness); + + return nr_reclaimed * PAGE_SIZE; +} + __bpf_kfunc_end_defs(); BTF_KFUNCS_START(bpf_memcontrol_kfuncs) @@ -174,19 +244,35 @@ BTF_ID_FLAGS(func, bpf_mem_cgroup_flush_stats, KF_SLEEPABLE) BTF_KFUNCS_END(bpf_memcontrol_kfuncs) +BTF_KFUNCS_START(bpf_memcontrol_reclaim_kfuncs) +BTF_ID_FLAGS(func, bpf_proactive_reclaim, KF_SLEEPABLE) +BTF_KFUNCS_END(bpf_memcontrol_reclaim_kfuncs) + static const struct btf_kfunc_id_set bpf_memcontrol_kfunc_set = { .owner = THIS_MODULE, .set = &bpf_memcontrol_kfuncs, }; +static const struct btf_kfunc_id_set bpf_memcontrol_reclaim_kfunc_set = { + .owner = THIS_MODULE, + .set = &bpf_memcontrol_reclaim_kfuncs, +}; + static int __init bpf_memcontrol_init(void) { int err; err = register_btf_kfunc_id_set(BPF_PROG_TYPE_UNSPEC, &bpf_memcontrol_kfunc_set); - if (err) + if (err) { pr_warn("error while registering bpf memcontrol kfuncs: %d", err); + return err; + } + + err = register_btf_kfunc_id_set(BPF_PROG_TYPE_SYSCALL, + &bpf_memcontrol_reclaim_kfunc_set); + if (err) + pr_warn("error registering bpf reclaim kfuncs: %d", err); return err; } -- 2.43.0
