linux-kernel - Re: [PATCH 4/5] powernv:idle: Move initialization of sibling pacas to pnv_alloc_idle_core

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Date:   Fri, 7 Jul 2017 20:34:16 +0530
From:   Gautham R Shenoy <ego@...ux.vnet.ibm.com>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michael Neuling <mikey@...ling.org>,
        Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
        Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Akshay Adiga <akshay.adiga@...ux.vnet.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH 4/5] powernv:idle: Move initialization of sibling pacas
 to pnv_alloc_idle_core_states

On Fri, Jul 07, 2017 at 01:16:09AM +1000, Nicholas Piggin wrote:
> On Wed,  5 Jul 2017 22:08:15 +0530
> "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com> wrote:
> 
> > From: "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
> > 
> > On POWER9 DD1, in order to get around a hardware issue, we store in
> > every CPU thread's paca the paca pointers of all its siblings.
> > 
> > Move this code into pnv_alloc_idle_core_states() soon after the space
> > for saving the sibling pacas is allocated.
> > 
> > Signed-off-by: Gautham R. Shenoy <ego@...ux.vnet.ibm.com>
> 
> > -	if (cpu_has_feature(CPU_FTR_POWER9_DD1)) {
> > -		int cpu;
> > -
> > -		pr_info("powernv: idle: Saving PACA pointers of all CPUs in their thread sibling PACA\n");
> > -		for_each_possible_cpu(cpu) {
> > -			int base_cpu = cpu_first_thread_sibling(cpu);
> > -			int idx = cpu_thread_in_core(cpu);
> > -			int i;
> > -
> 
> You could move the thread_sibling_pacas allocation to here?
> 
> Speaking of which... core_idle_state and thread_sibling_pacas are
> allocated with kmalloc_node... What happens if we take an SLB miss
> in the idle wakeup code on these guys? Nothing good I think. Perhaps
> we should put them into the pacas or somewhere in bolted memory.

Yes, though the SLB miss hasn't yet been encountered in practise so
far!

While one can define thread_sibling_pacas in PACA, it doesn't make
sense to allocate space for core_idle_state in PACA since the
allocated value of the secondary threads will never be used.

What is the right way to ensure that these allocations fall in the
bolted range ?

> 
> Good cleanup though.
> 
> Reviewed-by: Nicholas Piggin <npiggin@...il.com>
>