Hi, On 6/28/2023 8:59 AM, Alexei Starovoitov wrote: > On 6/26/23 12:16 AM, Hou Tao wrote: >> Hi, >> >> On 6/26/2023 12:42 PM, Alexei Starovoitov wrote: >>> On Sun, Jun 25, 2023 at 8:30 PM Hou Tao <houtao@xxxxxxxxxxxxxxx> wrote: >>>> Hi, >>>> >>>> On 6/24/2023 11:13 AM, Alexei Starovoitov wrote: >>>>> From: Alexei Starovoitov <ast@xxxxxxxxxx> >>>>> >>>>> alloc_bulk() can reuse elements from free_by_rcu_ttrace. >>>>> Let it reuse from waiting_for_gp_ttrace as well to avoid >>>>> unnecessary kmalloc(). >>>>> >>>>> Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx> >>>>> --- >>>>> kernel/bpf/memalloc.c | 9 +++++++++ >>>>> 1 file changed, 9 insertions(+) >>>>> SNIP >> // free A (from c1), ..., last free X (allocated from c0) >> P3: unit_free(c1) >> // the last freed element X is from c0 >> c1->tgt = c0 >> c1->free_llist->first -> X -> Y -> ... -> A >> P3: free_bulk(c1) >> enque_to_free(c0) >> c0->free_by_rcu_ttrace->first -> A -> ... -> Y -> X >> __llist_add_batch(c0->waiting_for_gp_ttrace) >> c0->waiting_for_gp_ttrace = A -> ... -> Y -> X > > In theory that's possible, but for this to happen one cpu needs > to be thousand times slower than all others and since there is no > preemption in llist_del_first I don't think we need to worry about it. Not sure whether or not such case will be possible in a VM, after all, the CPU X is just a thread in host and it may be preempted in any time and with any duration. > Also with removal of _tail optimization the above > llist_add_batch(waiting_for_gp_ttrace) > will become a loop, so reused element will be at the very end > instead of top, so one cpu to million times slower which is not > realistic. It is still possible A will be added back as waiting_for_gp_ttrace->first after switching to llist_add() as shown below. My questions is how much is the benefit for reusing from waiting_for_gp_ttrace ? // free A (from c1), ..., last free X (allocated from c0) P3: unit_free(c1) // the last freed element X is allocated from c0 c1->tgt = c0 c1->free_llist->first -> A -> ... -> Y c1->free_llist_extra -> X P3: free_bulk(c1) enque_to_free(c0) c0->free_by_rcu_ttrace->first -> Y -> ... A c0->free_by_rcu_ttrace->first -> X -> Y -> ... A llist_add(c0->waiting_for_gp_ttrace) c0->waiting_for_gp_ttrace = A -> .. -> Y -> X > >> P1: >> // A is added back as first again >> // but llist_del_first() didn't know >> try_cmpxhg(&c0->waiting_for_gp_ttrace->first, A, B) >> // c0->waiting_for_gp_trrace is corrupted >> c0->waiting_for_gp_ttrace->first = B >>
