>>> On Tue, Jun 24, 2008 at 8:24 AM, in message <1214310299.4351.27.camel@twins>, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > On Tue, 2008-06-24 at 07:15 -0600, Gregory Haskins wrote: >> >>> On Tue, Jun 24, 2008 at 6:13 AM, in message > <1214302405.4351.21.camel@twins>, >> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: >> > On Mon, 2008-06-23 at 17:04 -0600, Gregory Haskins wrote: >> >> We do find_busiest_groups() et. al. without locks held for normal > balancing, >> >> so lets do it for newidle as well. It will allow other cpus to make >> >> forward progress (against our RQ) while we try to balance and allow >> >> some interrupts to occur. >> > >> > Is running f_b_g really that expensive? >> >> According to our oprofile data, yes. I speculate that it works out that way > because most newidle >> attempts result in "no imbalance". But we were spending ~60%+ time in > find_busiest_groups() >> because of all the heavy-context switching that goes on in PREEMPT_RT. So > while f_b_g() is >> probably cheaper than double-lock/move_tasks(), the ratio of occurrence is > off the charts in >> comparison. Prior to this patch, those occurrences were > preempt-disabled/irq-disabled/rq->lock critical >> sections. >> >> So while it is not clear if f_b_g() is the actual cost, it is a convenient > (and legal, afaict) place to >> deterministically reduce the rq->lock scope. Additionally, doing so > measurably helps >> performance, so I think its a win. Without this patch you have to hope the > double_lock releases >> this_rq, and even so were not checking for the NEEDS_RESCHED. > > See, having had this information in the changelog to begin with would > have helped ;-) What? You can't read my mind? :) Good point, Peter. Will fix on next drop. -Greg > > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
