From 7cb685982157567dcc55eb92d1c38d237465203b Mon Sep 17 00:00:00 2001 From: Ingo Molnar Date: Mon, 18 May 2015 12:05:13 +0200 Subject: [PATCH] x86/smp/boot: Fix legacy SMP bootup slow-boot bug MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit So while testing kernels using tools/kvm/ (kvmtool) I noticed that it booted super slow: [ 0.142991] Performance Events: no PMU driver, software events only. [ 0.149265] x86: Booting SMP configuration: [ 0.149765] .... node #0, CPUs: #1 [ 0.148304] kvm-clock: cpu 1, msr 2:1bfe9041, secondary cpu clock [ 10.158813] KVM setup async PF for cpu 1 [ 10.159000] #2 [ 10.159000] kvm-stealtime: cpu 1, msr 211a4d400 [ 10.158829] kvm-clock: cpu 2, msr 2:1bfe9081, secondary cpu clock [ 20.167805] KVM setup async PF for cpu 2 [ 20.168000] #3 [ 20.168000] kvm-stealtime: cpu 2, msr 211a8d400 [ 20.167818] kvm-clock: cpu 3, msr 2:1bfe90c1, secondary cpu clock [ 30.176902] KVM setup async PF for cpu 3 [ 30.177000] #4 [ 30.177000] kvm-stealtime: cpu 3, msr 211acd400 One CPU booted up per 10 seconds. With 120 CPUs that takes a while. Bisection pinpointed this commit: 853b160aaafb ("Revert f5d6a52f5111 ("x86/smpboot: Skip delays during SMP initialization similar to Xen")") But that commit just restores previous behavior, so it cannot cause the problem. After some head scratching it turns out that these two commits: 1a744cb356c5 ("x86/smp/boot: Remove 10ms delay from cpu_up() on modern processors") d68921f9bd14 ("x86/smp/boot: Add cmdline "cpu_init_udelay=N" to specify cpu_up() delay") added the following code to smpboot.c: - mdelay(10); + mdelay(init_udelay); Note the mismatch in the units: the delay is called 'udelay' and is set to microseconds - while the function used here is actually 'mdelay', which counts in milliseconds ... So the delay for legacy systems is off by a factor of 1,000, so instead of 10 msecs we waited for 10 seconds ... The reason bisection pointed to 853b160aaafb was that 853b160aaafb removed a (broken) boot-time speedup patch, which masked the factor 1,000 bug. Fix it by using udelay(). This fixes my bootup problems. Cc: Len Brown Cc: Alan Cox Cc: Arjan van de Ven Cc: Borislav Petkov Cc: H. Peter Anvin Cc: Jan H. Schönherr Cc: Linus Torvalds Cc: Thomas Gleixner Cc: linux-kernel@vger.kernel.org Signed-off-by: Ingo Molnar --- arch/x86/kernel/smpboot.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c index b9aaa3930b2f..fd6291c921b6 100644 --- a/arch/x86/kernel/smpboot.c +++ b/arch/x86/kernel/smpboot.c @@ -617,7 +617,7 @@ wakeup_secondary_cpu_via_init(int phys_apicid, unsigned long start_eip) pr_debug("Waiting for send to finish...\n"); send_status = safe_apic_wait_icr_idle(); - mdelay(init_udelay); + udelay(init_udelay); pr_debug("Deasserting INIT\n"); -- 2.20.1