On Tue, Feb 28, 2012 at 1:41 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: > On Tue, 28 Feb 2012, Peter Zijlstra wrote: > >> On Mon, 2012-02-27 at 12:38 -0800, Dan Williams wrote: >> > An experimental hack to tease out whether we are continuing to >> > run the softirq handler past the point of needing scheduling. >> > >> > It allows only one trip through __do_softirq() as long as need_resched() >> > is set which hopefully creates the back pressure needed to get ksoftirqd >> > scheduled. >> > >> > Targeted to address reports like the following that are produced >> > with i/o tests to a sas domain with a large number of disks (48+), and >> > lots of debugging enabled (slub_deubg, lockdep) that makes the >> > block+scsi softirq path more cpu-expensive than normal. >> > >> > With this patch applied the softlockup detector seems appeased, but it >> > seems odd to need changes to kernel/softirq.c so maybe I have overlooked >> > something that needs changing at the block/scsi level? >> > >> > BUG: soft lockup - CPU#3 stuck for 22s! [kworker/3:1:78] >> >> So you're stuck in softirq for 22s+, max_restart is 10, this gives that >> on average you spend 2.2s+ per softirq invocation, this is completely >> absolutely bonkers. Softirq handlers should never consume significant >> amount of cpu-time. >> >> Thomas, think its about time we put something like the below in? > > Absolutely. Anything which consumes more than a few microseconds in > the softirq handler needs to be sorted out, no matter what. Looks like everyone is guilty: [ 422.765336] softirq took longer than 1/4 tick: 3 NET_RX ffffffff813f0aa0 ... [ 423.971878] softirq took longer than 1/4 tick: 4 BLOCK ffffffff812519c8 [ 423.985093] softirq took longer than 1/4 tick: 6 TASKLET ffffffff8103422e [ 423.993157] softirq took longer than 1/4 tick: 7 SCHED ffffffff8105e2e1 [ 424.001018] softirq took longer than 1/4 tick: 9 RCU ffffffff810a0fed [ 424.008691] softirq loop took longer than 1/2 tick need_resched: yes max_restart: 10 As expected whenever that 1/2 tick message gets emitted the softirq handler is almost running in a need_resched() context. $ grep need_resched.*no log | wc -l 295 $ grep need_resched.*yes log | wc -l 3201 One of these warning messages gets printed out at a rate of 1 every 40ms. (468 second log w/ 11,725 of these messages). So is it a good idea to get more aggressive about scheduling ksoftrrqd? -- Dan -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
