On Sun, Jun 05, 2022 at 11:10:31AM +0200, Uladzislau Rezki wrote: > > On Thu, Jun 02, 2022 at 10:06:44AM +0200, Uladzislau Rezki (Sony) wrote: > > > Currently the monitor work is scheduled with a fixed interval that > > > is HZ/20 or each 50 milliseconds. The drawback of such approach is > > > a low utilization of page slot in some scenarios. The page can store > > > up to 512 records. For example on Android system it can look like: > > > > > > <snip> > > > kworker/3:0-13872 [003] .... 11286.007048: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000026522604 nr_records=1 > > > kworker/3:0-13872 [003] .... 11286.015638: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000095ed6fca nr_records=2 > > > kworker/1:2-20434 [001] .... 11286.051230: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000044872ffd nr_records=1 > > > kworker/1:2-20434 [001] .... 11286.059322: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000026522604 nr_records=2 > > > kworker/0:1-20052 [000] .... 11286.095295: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000044872ffd nr_records=2 > > > kworker/0:1-20052 [000] .... 11286.103418: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x00000000cbcf05db nr_records=1 > > > kworker/2:3-14372 [002] .... 11286.135155: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000095ed6fca nr_records=2 > > > kworker/2:3-14372 [002] .... 11286.135198: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000044872ffd nr_records=1 > > > kworker/1:2-20434 [001] .... 11286.155377: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x00000000cbcf05db nr_records=5 > > > kworker/2:3-14372 [002] .... 11286.167181: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000026522604 nr_records=5 > > > kworker/1:2-20434 [001] .... 11286.179202: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x000000008ef95e14 nr_records=1 > > > kworker/2:3-14372 [002] .... 11286.187398: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x00000000c597d297 nr_records=6 > > > kworker/3:0-13872 [003] .... 11286.187445: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000050bf92e2 nr_records=3 > > > kworker/1:2-20434 [001] .... 11286.198975: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x00000000cbcf05db nr_records=4 > > > kworker/1:2-20434 [001] .... 11286.207203: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000095ed6fca nr_records=4 > > > <snip> > > > > > > where a page only carries few records to reclaim a memory. In order to > > > improve batching and make utilization more efficient the patch introduces > > > a drain interval that can be set either to KFREE_DRAIN_JIFFIES_MAX or > > > KFREE_DRAIN_JIFFIES_MIN. It is adjusted if a flood is detected, in this > > > case a memory reclaim occurs more often whereas in mostly idle cases the > > > interval is set to its maximum timeout that improves the utilization of > > > page slots. > > > > > > Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx> > > > > That does look like a problem well worth solving! > > > Agree, better ideas make better final solution :) > > > > > But I am missing one thing. If we are having a callback flood, why do we > > need a shorter timeout? > > > To offload faster, because otherwise we run into classical issue, it is a low > memory condition state resulting in OOM. But doesn't each callback queued during the flood give us an opportunity to react to the flood? That will be way more fine-grained than any reasonable timer, right? Or am I missing something? I do agree that the action would often need to be indirect to avoid the memory-allocation-state hassles, but we already can do that, either via an extremely short-term hrtimer or something like irq-work. > > Wouldn't a check on the number of blocks queued be simpler, more direct, > > and provide faster response to the start of a callback flood? > > > I rely on krcp->count because not always we can store the pointer in the page > slots. We can not allocate a page in the caller context thus we use page-cache > worker that fills the cache in normal context. While it populates the cache, > pointers temporary are queued to the linked-list. > > Any thoughts? There are a great many ways to approach this. One of them is to maintain a per-CPU free-running counter of kvfree_rcu() calls, and to reset this counter each jiffy. Or am I missing a trick here? Thanx, Paul
