Re: [PATCH v1 1/1] crypto/pcrypt: Do not use isolated CPUs for callback

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, Oct 11, 2022 at 03:20:39PM -0300, Leonardo Brás wrote:
> On Fri, 2022-10-07 at 18:42 -0300, Leonardo Brás wrote:
> > On Wed, 2022-10-05 at 09:57 -0300, Marcelo Tosatti wrote:
> > > On Tue, Oct 04, 2022 at 03:25:37AM -0300, Leonardo Bras wrote:
> > > > Currently pcrypt_aead_init_tfm() will pick callback cpus (ctx->cb_cpu)
> > > > from any online cpus. Later padata_reorder() will queue_work_on() the
> > > > chosen cb_cpu.
> > > > 
> > > > This is undesired if the chosen cb_cpu is listed as isolated (i.e. using
> > > > isolcpus=... kernel parameter), since the work queued will interfere with
> > > > the workload on the isolated cpu.
> > > > 
> > > > Make sure isolated cpus are not used for pcrypt.
> > > > 
> > > > Signed-off-by: Leonardo Bras <leobras@redhat.com>
> > > > ---
> > > >  crypto/pcrypt.c | 10 +++++++---
> > > >  1 file changed, 7 insertions(+), 3 deletions(-)
> > > > 
> > > > diff --git a/crypto/pcrypt.c b/crypto/pcrypt.c
> > > > index 9d10b846ccf73..9017d08c91a8d 100644
> > > > --- a/crypto/pcrypt.c
> > > > +++ b/crypto/pcrypt.c
> > > > @@ -16,6 +16,7 @@
> > > >  #include <linux/kobject.h>
> > > >  #include <linux/cpu.h>
> > > >  #include <crypto/pcrypt.h>
> > > > +#include <linux/sched/isolation.h>
> > > >  
> > > >  static struct padata_instance *pencrypt;
> > > >  static struct padata_instance *pdecrypt;
> > > > @@ -175,13 +176,16 @@ static int pcrypt_aead_init_tfm(struct crypto_aead *tfm)
> > > >  	struct pcrypt_instance_ctx *ictx = aead_instance_ctx(inst);
> > > >  	struct pcrypt_aead_ctx *ctx = crypto_aead_ctx(tfm);
> > > >  	struct crypto_aead *cipher;
> > > > +	struct cpumask non_isolated;
> > > > +
> > > > +	cpumask_and(&non_isolated, cpu_online_mask, housekeeping_cpumask(HK_TYPE_DOMAIN));
> > > 
> > > Since certain systems do not use isolcpus=domain, so please use a flag
> > > that is setup by nohz_full=, for example HK_FLAG_MISC:
> > > 
> > > static int __init housekeeping_nohz_full_setup(char *str)
> > > {
> > >         unsigned long flags;
> > > 
> > >         flags = HK_FLAG_TICK | HK_FLAG_WQ | HK_FLAG_TIMER | HK_FLAG_RCU |
> > >                 HK_FLAG_MISC | HK_FLAG_KTHREAD;
> > > 
> > >         return housekeeping_setup(str, flags);
> > > }
> > > __setup("nohz_full=", housekeeping_nohz_full_setup);
> > 
> > Oh, sure. 
> > Since we are talking about WorkQueues, I think it makes sense to pick
> > HK_FLAG_WQ. 
> > 
> > > 
> > > Also, shouldnt you use cpumask_t ?/
> > 
> > Yeah, I think so. 
> > I was quick to choose the 'struct cpumask' because all functions would operate
> > in this variable type, but yeah, I think it makes sense to have this variable
> > being opaque here.
> 
> In fact, it seems neither 'struct cpumask' nor 'cpumask_t' are recommended to be
> used allocated in the stack, due to the large size it can get (up to 1kB). 
> 
> At include/linux/cpumask.h we have:
> 'cpumask_var_t: struct cpumask for stack usage'
> which should work better at least for init functions like this.
> 
> In other cases, I see 'static cpumask_t' being used to avoid the allocation
> overhead, but it's probably due to the functions being called in very specific
> scenarios. It could mean trouble if multiple cpus try to use it at once.
> 
> What do you recommend on it?

Sorry for the delay. I suppose allocating and freeing is OK in this context, since
its initialization time and not a hot path?