Re: [PATCH v3 bpf-next 1/2] bpf: Allow bpf_local_storage to be used by sleepable programs
From: Martin KaFai Lau <hidden>
Date: 2021-12-28 23:58:29
On Fri, Dec 24, 2021 at 03:29:15PM +0000, KP Singh wrote:
Other maps like hashmaps are already available to sleepable programs.
Sleepable BPF programs run under trace RCU. Allow task, sk and inode
storage to be used from sleepable programs. This allows sleepable and
non-sleepable programs to provide shareable annotations on kernel
objects.
Sleepable programs run in trace RCU where as non-sleepable programs run
in a normal RCU critical section i.e. __bpf_prog_enter{_sleepable}
and __bpf_prog_exit{_sleepable}) (rcu_read_lock or rcu_read_lock_trace).
In order to make the local storage maps accessible to both sleepable
and non-sleepable programs, one needs to call both
call_rcu_tasks_trace and call_rcu to wait for both trace and classical
RCU grace periods to expire before freeing memory.
Paul's work on call_rcu_tasks_trace allows us to have per CPU queueing
for call_rcu_tasks_trace. This behaviour can be achieved by setting
rcupdate.rcu_task_enqueue_lim=<num_cpus> boot parameter.
In light of these new performance changes and to keep the local storage
code simple, avoid adding a new flag for sleepable maps / local storage
to select the RCU synchronization (trace / classical).
Also, update the dereferencing of the pointers to use
rcu_derference_check (with either the trace or normal RCU locks held)
with a common bpf_rcu_lock_held helper method.
Signed-off-by: KP Singh <kpsingh@kernel.org>Thanks for the patches. One minor comment which is not very related to this set. We can land this set first and then use a follow up. Acked-by: Martin KaFai Lau <redacted>
quoted hunk ↗ jump to hunk
@@ -306,7 +328,8 @@ int bpf_local_storage_alloc(void *owner, * bucket->list, first_selem can be freed immediately * (instead of kfree_rcu) because * bpf_local_storage_map_free() does a - * synchronize_rcu() before walking the bucket->list. + * synchronize_rcu_mult (waiting for both sleepable and + * normal programs) before walking the bucket->list. * Hence, no one is accessing selem from the * bucket->list under rcu_read_lock(). */
This whole comment section is outdated and can be removed because the bpf_free_used_maps(bpf_prog->aux) is now only called after call_rcu_tasks_trace() or call_rcu(). Meaning bpf_local_storage_map_free() cannot be running in parallel with bpf_local_storage_alloc() because the running bpf prog holds a refcnt of the smap here.