Re: [Xen-devel] [PATCH 2/2] x86/time: update vtsc_last with cmpxchg and drop vtsc_lock

[Jan Beulich <jbeulich@xxxxxxxx>]

On 16.12.2019 11:00, Roger Pau Monné wrote:
> On Fri, Dec 13, 2019 at 10:48:02PM +0000, Igor Druzhinin wrote:
>> Now that vtsc_last is the only entity protected by vtsc_lock we can
>> simply update it using a single atomic operation and drop the spinlock
>> entirely. This is extremely important for the case of running nested
>> (e.g. shim instance with lots of vCPUs assigned) since if preemption
>> happens somewhere inside the critical section that would immediately
>> mean that other vCPU stop progressing (and probably being preempted
>> as well) waiting for the spinlock to be freed.
>>
>> This fixes constant shim guest boot lockups with ~32 vCPUs if there is
>> vCPU overcommit present (which increases the likelihood of preemption).
>>
>> Signed-off-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
>> ---
>>  xen/arch/x86/domain.c        |  1 -
>>  xen/arch/x86/time.c          | 16 ++++++----------
>>  xen/include/asm-x86/domain.h |  1 -
>>  3 files changed, 6 insertions(+), 12 deletions(-)
>>
>> diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
>> index bed19fc..94531be 100644
>> --- a/xen/arch/x86/domain.c
>> +++ b/xen/arch/x86/domain.c
>> @@ -539,7 +539,6 @@ int arch_domain_create(struct domain *d,
>>      INIT_PAGE_LIST_HEAD(&d->arch.relmem_list);
>>  
>>      spin_lock_init(&d->arch.e820_lock);
>> -    spin_lock_init(&d->arch.vtsc_lock);
>>  
>>      /* Minimal initialisation for the idle domain. */
>>      if ( unlikely(is_idle_domain(d)) )
>> diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
>> index 216169a..202446f 100644
>> --- a/xen/arch/x86/time.c
>> +++ b/xen/arch/x86/time.c
>> @@ -2130,19 +2130,15 @@ u64 gtsc_to_gtime(struct domain *d, u64 tsc)
>>  
>>  uint64_t pv_soft_rdtsc(const struct vcpu *v, const struct cpu_user_regs 
>> *regs)
>>  {
>> -    s_time_t now = get_s_time();
>> +    s_time_t old, new, now = get_s_time();
>>      struct domain *d = v->domain;
>>  
>> -    spin_lock(&d->arch.vtsc_lock);
>> -
>> -    if ( (int64_t)(now - d->arch.vtsc_last) > 0 )
>> -        d->arch.vtsc_last = now;
>> -    else
>> -        now = ++d->arch.vtsc_last;
>> -
>> -    spin_unlock(&d->arch.vtsc_lock);
>> +    do {
>> +        old = d->arch.vtsc_last;
>> +        new = (int64_t)(now - d->arch.vtsc_last) > 0 ? now : old + 1;
> 
> Why do you need to do this subtraction? Isn't it easier to just do:
> 
> new = now > d->arch.vtsc_last ? now : old + 1;

This wouldn't be reliable when the TSC wraps. Remember that firmware
may set the TSC, and it has been seen to be set to very large
(effectively negative, if they were signed quantities) values, which
will then eventually wrap (whereas we're not typically concerned of
64-bit counters wrapping when they start from zero).

Jan

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