On 8/28/24 13:09, Uladzislau Rezki (Sony) wrote: > Add a support of dynamically attaching an rcu_head to an object > which gets freed via the single argument of kvfree_rcu(). This is > used in the path, when a page allocation fails due to a high memory > pressure. > > The basic idea behind of this is to minimize a hit of slow path > which requires a caller to wait until a grace period is passed. > > Signed-off-by: Uladzislau Rezki (Sony) <ure...@gmail.com>
So IIUC it's a situation where we can't allocate a page, but we hope the kmalloc-32 slab has still free objects to give us dyn_rcu_head's before it would have to also make a page allocation? So that may really be possible and there might potentially be many such objects, but I wonder if there's really a benefit. The system is struggling for memory and the single-argument caller specifically is _mightsleep so it could e.g. instead go direct reclaim a page rather than start depleting the kmalloc-32 slab, no? > --- > kernel/rcu/tree.c | 53 +++++++++++++++++++++++++++++++++++++++++++---- > 1 file changed, 49 insertions(+), 4 deletions(-) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index be00aac5f4e7..0124411fecfb 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -3425,6 +3425,11 @@ kvfree_rcu_bulk(struct kfree_rcu_cpu *krcp, > cond_resched_tasks_rcu_qs(); > } > > +struct dyn_rcu_head { > + unsigned long *ptr; > + struct rcu_head rh; > +}; > + > static void > kvfree_rcu_list(struct rcu_head *head) > { > @@ -3433,15 +3438,32 @@ kvfree_rcu_list(struct rcu_head *head) > for (; head; head = next) { > void *ptr = (void *) head->func; > unsigned long offset = (void *) head - ptr; > + struct dyn_rcu_head *drhp = NULL; > + > + /* > + * For dynamically attached rcu_head, a ->func field > + * points to _offset_, i.e. not to a pointer which has > + * to be freed. For such objects, adjust an offset and > + * pointer. > + */ > + if (__is_kvfree_rcu_offset((unsigned long) ptr)) { > + drhp = container_of(head, struct dyn_rcu_head, rh); > + offset = (unsigned long) drhp->rh.func; > + ptr = drhp->ptr; > + } > > next = head->next; > debug_rcu_head_unqueue((struct rcu_head *)ptr); > rcu_lock_acquire(&rcu_callback_map); > trace_rcu_invoke_kvfree_callback(rcu_state.name, head, offset); > > - if (!WARN_ON_ONCE(!__is_kvfree_rcu_offset(offset))) > + if (!WARN_ON_ONCE(!__is_kvfree_rcu_offset(offset))) { > kvfree(ptr); > > + if (drhp) > + kvfree(drhp); > + } > + > rcu_lock_release(&rcu_callback_map); > cond_resched_tasks_rcu_qs(); > } > @@ -3787,6 +3809,21 @@ add_ptr_to_bulk_krc_lock(struct kfree_rcu_cpu **krcp, > return true; > } > > +static struct rcu_head * > +attach_rcu_head_to_object(void *obj) > +{ > + struct dyn_rcu_head *rhp; > + > + rhp = kmalloc(sizeof(struct dyn_rcu_head), GFP_KERNEL | > + __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN); > + > + if (!rhp) > + return NULL; > + > + rhp->ptr = obj; > + return &rhp->rh; > +} > + > /* > * Queue a request for lazy invocation of the appropriate free routine > * after a grace period. Please note that three paths are maintained, > @@ -3830,9 +3867,17 @@ void kvfree_call_rcu(struct rcu_head *head, void *ptr) > if (!success) { > run_page_cache_worker(krcp); > > - if (head == NULL) > - // Inline if kvfree_rcu(one_arg) call. > - goto unlock_return; > + if (!head) { > + krc_this_cpu_unlock(krcp, flags); > + head = attach_rcu_head_to_object(ptr); > + krcp = krc_this_cpu_lock(&flags); > + > + if (!head) > + // Inline if kvfree_rcu(one_arg) call. > + goto unlock_return; > + > + ptr = (rcu_callback_t) offsetof(struct dyn_rcu_head, > rh); > + } > > head->func = ptr; > head->next = krcp->head;