linux-kernel - Re: [PATCH] x86,cpu-hotplug: assign same CPU number to readded CPU

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Message-ID: <20140711105920.GC17083@pd.tnic>
Date:	Fri, 11 Jul 2014 12:59:20 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
Cc:	Igor Mammedov <imammedo@...hat.com>, tglx@...utronix.de,
	mingo@...hat.com, hpa@...or.com, x86@...nel.org, toshi.kani@...com,
	huawei.libin@...wei.com, paul.gortmaker@...driver.com,
	linux-kernel@...r.kernel.org, srivatsa.bhat@...ux.vnet.ibm.com,
	umgwanakikbuti@...il.com, peterz@...radead.org
Subject: Re: [PATCH] x86,cpu-hotplug: assign same CPU number to readded CPU

On Fri, Jul 11, 2014 at 09:48:27AM +0900, Yasuaki Ishimatsu wrote:
> >>+static int get_cpuid(int apicid)

Btw this "cpuid" is misleading. Call it "cpu_num" or so.

> >>+{
> >>+	int cpuid;
> >>+
> >>+	cpuid = apicid_to_cpuid[apicid];
> >>+	if (cpuid < 0)
> >>+		cpuid = cpumask_next_zero(-1, cpu_used_mask);
> >Why do you need additional cpu bitmask?
> 
> To assing new CPU number, I prepared new cpu bitmask.
> 
> The following two steps are necessary to assign CPU number to APIC ID.
>  1. Check whether APIC ID has been assigned CPU number
>  2. Assign new CPU number if ACPI ID has not been assigned CPU number (it
>     means apicid_to_cpuid[] returns -1)
> 
> Step 1. is checked by apicid_to_cpuid[]. And step 2. assigns new CPU
> number by using cpu_used_mask.
> 
> To keep cpu number, cpumask must not be cleared by hot removing CPU.
> If cpumask is cleared by hot removing CPU, the cpumask cannot be used
> to keep CPU number.
> 
> Currently, cpu_present_map is used to assign CPU number. But the cpumask
> is cleared by hot removing CPU since the mask is prepared to remember
> existed CPUs in the system. So the cpu_present_map must be cleared
> at CPU hot remove.
> 
> I confirmed whether present cpumasks (cpu_possible_map, cpu_online_map
> et al) is usable or not for this purpose. But there is no cpumask that
> can be used to keep CPU number. So I prepared new cpu bitmask.
> 
> >How about just finding the first apicid_to_cpuid[apicid] < 0
> >and dropping not needed anymore bitmask.
> 
> When apicid_to_cpuid[] return -1, kernel assigns new CPU number. For
> this, the cpu_used_mask is necessary.

And we can't have that - we cannot have cores which had number X get
number Y after hotplug.

Can you send a full dmesg after you've done a physical node hotplug on a
machine? Privately is fine too.

Boot with

	"ignore_loglevel log_buf_len=10M debug apic=debug show_lapic=all"

please.

Thanks.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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