Re: [PATCH 1/2] s390/gmap: voluntarily schedule during key setting

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Mon, 30 May 2022 11:27:05 +0200
Christian Borntraeger <borntraeger@linux.ibm.com> wrote:

> With large and many guest with storage keys it is possible to create
> large latencies or stalls during initial key setting:
> 
> rcu: INFO: rcu_sched self-detected stall on CPU
> rcu:   18-....: (2099 ticks this GP) idle=54e/1/0x4000000000000002 softirq=35598716/35598716 fqs=998
>        (t=2100 jiffies g=155867385 q=20879)
> Task dump for CPU 18:
> CPU 1/KVM       R  running task        0 1030947 256019 0x06000004
> Call Trace:
> sched_show_task
> rcu_dump_cpu_stacks
> rcu_sched_clock_irq
> update_process_times
> tick_sched_handle
> tick_sched_timer
> __hrtimer_run_queues
> hrtimer_interrupt
> do_IRQ
> ext_int_handler
> ptep_zap_key
> 
> The mmap lock is held during the page walking but since this is a
> semaphore scheduling is still possible. Same for the kvm srcu.
> To minimize overhead do this on every segment table entry or large page.
> 
> Signed-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>

> ---
>  arch/s390/mm/gmap.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
> index 1ac73917a8d3..b8ae4a4aa2ba 100644
> --- a/arch/s390/mm/gmap.c
> +++ b/arch/s390/mm/gmap.c
> @@ -2608,6 +2608,18 @@ static int __s390_enable_skey_pte(pte_t *pte, unsigned long addr,
>  	return 0;
>  }
>  
> +/*
> + * Give a chance to schedule after setting a key to 256 pages.
> + * We only hold the mm lock, which is a rwsem and the kvm srcu.
> + * Both can sleep.
> + */
> +static int __s390_enable_skey_pmd(pmd_t *pmd, unsigned long addr,
> +				  unsigned long next, struct mm_walk *walk)
> +{
> +	cond_resched();
> +	return 0;
> +}
> +
>  static int __s390_enable_skey_hugetlb(pte_t *pte, unsigned long addr,
>  				      unsigned long hmask, unsigned long next,
>  				      struct mm_walk *walk)
> @@ -2630,12 +2642,14 @@ static int __s390_enable_skey_hugetlb(pte_t *pte, unsigned long addr,
>  	end = start + HPAGE_SIZE - 1;
>  	__storage_key_init_range(start, end);
>  	set_bit(PG_arch_1, &page->flags);
> +	cond_resched();
>  	return 0;
>  }
>  
>  static const struct mm_walk_ops enable_skey_walk_ops = {
>  	.hugetlb_entry		= __s390_enable_skey_hugetlb,
>  	.pte_entry		= __s390_enable_skey_pte,
> +	.pmd_entry		= __s390_enable_skey_pmd,
>  };
>  
>  int s390_enable_skey(void)