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Message-ID: <20151012005117.GA15270@lerouge> Date: Mon, 12 Oct 2015 02:51:19 +0200 From: Frederic Weisbecker <fweisbec@...il.com> To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> Cc: Meelis Roos <mroos@...ux.ee>, Linux Kernel list <linux-kernel@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de> Subject: Re: 4.2: CONFIG_NO_HZ_FULL_ALL effectively disabling non-boot CPUs On Sat, Oct 10, 2015 at 12:24:39PM -0700, Paul E. McKenney wrote: > On Sat, Oct 10, 2015 at 10:14:25PM +0300, Meelis Roos wrote: > > Short summary: turning on CONFIG_NO_HZ_FULL_ALL seems to disable all > > non-boot CPUs for scheduler. > > > > A couple of days ago I noticed that make -j8 on a 4-core i5 is very slow > > (with 4.3.0-rc4+git). Looking at top ('1' for per-cpu states), only > > first CPU is loaded and 3 other CPUs are 100% idle. This seems to be a > > problem on 3 of my desktop machines (different generation Intel: i5-660, > > i5-2400, i3-3220). All the computers run custom kernels. > > > > Further investigation showed that CPU affinity was set to 1 (CPU0 only) > > for init and all the children. Kernel threads had affinities 1,2,4,8 > > and f (seems normal). > > > > Even more interesting was the behaviour after setting affinity to f for > > all userland processes and then running make -j4. The other cores were > > still idle! > > > > Switching back to 4.2.0 with my config, the problem persisted. 4.2.3 as > > packaged by Debian worked fine. 4.0.0 and 4.1.0 with my config worked > > also fine. systemd and sysvinit behaved the same and no affinity was > > configured for systemd. > > > > So did a kernel config bisection between my kernel config and Debian > > config and came to CONFIG_NO_HZ_FULL_ALL. Debian has it off, I had it > > on. Turning that off fixed the scheduling and the system spread the > > tasks to all the cores. > > > > I do not remember changing this value for a long time, I set them after > > the settings were introduced and used it. So it seems it broken in 4.2.0 > > but was working in 4.1 but I do not have 4.1 config saved anywhere > > (many make oldconfigs since). > > > > Bisection between 4.1 and 4.2 is possible but not easy since the > > machines are usually actively used when I am near them. > > This is expected and intended behavior. The whole point of > CONFIG_NO_HZ_FULL_ALL is to keep everything off of the non-boot CPUs > that is not explicitly placed there. Without CONFIG_NO_HZ_FULL_ALL, > you can use the nohz_full boot parameter to select exactly which > CPUs are to behave this way. I'm preparing a revert of this. Many people are complaining about this. Most of the time it's about accidentally enbling NO_HZ_FULL_ALL and I could fix this with a warning to avoid time spent by users to chase a non-bug. But Mike says that CONFIG_NO_HZ_FULL_ALL makes the machine unusable for anything else than isolation workloads whereas some "normal" workload may be needed as well by the machine before or after an isolation task. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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