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X-MS-Exchange-CrossTenant-Network-Message-Id: 41649cab-fbd1-4f16-949f-08dd2df2e7de X-MS-Exchange-CrossTenant-AuthSource: BN0PR10MB5128.namprd10.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 06 Jan 2025 01:39:05.3053 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 4e2c6054-71cb-48f1-bd6c-3a9705aca71b X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: T0ikkV4W+ffBRiTVK/gyxth5HQ4vZGNpqPvmRXfB8mM0uy50At/rQKU//3IdloPXKCIRhJKipljhhiuVEQBpyA== X-MS-Exchange-Transport-CrossTenantHeadersStamped: IA0PR10MB7326 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1057,Hydra:6.0.680,FMLib:17.12.68.34 definitions=2025-01-02_03,2025-01-02_01,2024-11-22_01 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=0 adultscore=0 spamscore=0 mlxlogscore=886 bulkscore=0 phishscore=0 malwarescore=0 mlxscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2411120000 definitions=main-2501060012 X-Proofpoint-ORIG-GUID: ypEJdYAFbUGUrC8EyHNQBs36Raho1px_ X-Proofpoint-GUID: ypEJdYAFbUGUrC8EyHNQBs36Raho1px_ On 1/5/25 5:28 PM, NeilBrown wrote: > On Sun, 05 Jan 2025, Chuck Lever wrote: >> On 1/3/25 5:53 PM, Tejun Heo wrote: >>> On Sat, Jan 04, 2025 at 09:50:32AM +1100, NeilBrown wrote: >>>> On Fri, 03 Jan 2025, cel@kernel.org wrote: >>> ... >>>> I think that instead of passing "list_lru_count()" we should pass some >>>> constant like 1024. >>>> >>>> cnt = list_lru_count() >>>> while (cnt > 0) { >>>> num = min(cnt, 1024); >>>> list_lru_walk(...., num); >>>> cond_sched() >>>> cnt -= num; >>>> } >>>> >>>> Then run it from system_wq. >>>> >>>> list_lru_shrink is most often called as list_lru_shrink_walk() from a >>>> shrinker, and the pattern there is essentially that above. A count is >>>> taken, possibly scaled down, then the shrinker is called in batches. >>> >>> BTW, there's nothing wrong with taking some msecs or even tens of msecs >>> running on system_unbound_wq, so the current state may be fine too. >> >> My thinking was that this work is low priority, so there should be >> plenty of opportunity to set it aside for a few moments and handle >> higher priority work. Maybe not worrisome on systems with a high core >> count, but on small SMP (eg VM guests), I've found that tasks like this >> can be rude neighbors. > > None of the different work queues are expected to "set aside" the work > for more than a normal scheduling time slice. > system_long_wq was created "so that flush_scheduled_work() doesn't take > so long" but flush_scheduled_work() is never called now (and would > generate a warning if it was) so system_long_wq should really be > removed. > > system_unbound_wq uses threads that are not bound to a CPU so the > scheduler can move them around. That is most suitable for something > that might run for a long time. That's not my understanding: unbound selects a CPU to run the work item on, and it can be (and usually is) not the same CPU as the one that invoked queue_work(). Then it isn't rescheduled. The work items are expected to complete quickly; work item termination is the typical reschedule point. My understanding, as always, could be stale. But that's neither here nor there: I've dropped the patch to replace system_unbound_wq with system_long_wq. > system_wq is bound to the CPU that schedules the work, but it can run > multiple work items - potentially long running ones - without trouble by > helping the scheduler share the CPU among them. This requires that they > can sleep of course. > > I haven't seen the actually symptoms that resulted in the various > changes to this code, but that last point is I think the key one. The > work item, like all kernel code, needs to have scheduler points if it > might run for a longish time. list_lru_walk() doesn't contain any > scheduling points so giving a large "nr_to_walk" is always a bad idea. That might be a overstated... I don't see other consumers that are so concerned about rescheduling. But then it's not clear if they are dealing with lengthy LRU lists. >> We could do this by adding a cond_resched() call site in the loop, >> or take Neil's suggestion of breaking up the free list across multiple >> work items that handle one or just a few file releases each. > > I guess I should send a proper patch. More comments in reply to that patch. Generally speaking, I'm comfortable chopping up the work as you propose, we just have to address some details. -- Chuck Lever