On Sat, Jan 26, 2019 at 09:00:03PM +0000, Paul Walmsley wrote: > On Fri, 25 Jan 2019, Tony Lindgren wrote: > > > * Russell King <rmk+kernel@xxxxxxxxxxxxxxx> [190125 21:04]: > > > Executing loops such as: > > > > > > while (1) > > > cpu_relax(); > > > > > > with interrupts disabled results in a livelock of the entire system, > > > as other CPUs are prevented making progress. This is most noticable > > > as a failure of crashdump kexec, which stops just after issuing: > > > > > > Loading crashdump kernel... > > > > > > to the system console. A workaround for this is to use 10 nops in > > > cpu_relax(). > > > > > > We also use wfe() in while (1) loops to avoid burning cycles in a > > > tight loop, giving the CPU a hint that we're not doing anything > > > useful. > > > > > > Signed-off-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxx> > > > --- > > > It's been a while since this was posted, Will's suggestion was to use > > > 10 nops in cpu_relax() last time around. I still prefer wfe() in > > > these infinite-not-doing-anything-ever loops. > > > > Works for me: > > > > Tested-by: Tony Lindgren <tony@xxxxxxxxxxx> > > There was some concern in the past that WFE, like WFI, might cause the > core to assert an external signal that might cause the SoC integration to > place the core into a low-power mode from which it might not be able to > wake up. This could happen on OMAP, for example, with WFI. > > I don't recall the outcome of those discussions. Was a conclusion ever > reached? First, we use WFE in spinlocks. If WFE were to place the CPU in a low power state that it may not be able to wake up from, all our spinlocks would be unsafe. Next, in all of the situations in this patch, we're executing an infinite loop. If it were to cause the core to go into a low power mode, surely that's a good thing, rather than the core endlessly executing NOPs? The only way out of that is for the core to receive a reset _anyway_. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up According to speedtest.net: 11.9Mbps down 500kbps up
