Re: Issue: Can padata avoid dealing with CPU-related operations?

[Daniel Jordan]

On Sat, Oct 07, 2023 at 10:56:31AM +0800, Wang Jinchao wrote:
> On Wed, Oct 04, 2023 at 10:52:57AM -0400, Daniel Jordan wrote:
> > Hi,
> > 
> > On Wed, Sep 27, 2023 at 09:25:54PM +0800, Wang Jinchao wrote:
> > > Hello, I have a few questions about the padata code I've been studying
> > > recently:
> > > 
> > > - Why does padata use the WQ_UNBOUND attribute of the workqueue?
> > 
> > There's background in this series:
> >     https://lore.kernel.org/all/20190813005224.30779-1-daniel.m.jordan@xxxxxxxxxx/
> > 
> Summarizing, the use of the WQ_UNBOUND attribute is primarily based on performance
> considerations. Is this understanding correct?

Yes, it's entirely based on performance.  In the tests I ran, padata did
a lot better when it was free to run parallel works on any CPU in case
some were busy.

> > > Because I've noticed a significant maintenance cost related to CPUs.
> > > Are there any specific benefits?
> > 
> > Aside from what Steffen said about serialization, the pcrypt cpumasks
> > can be set from sysfs to control where parallel and serial jobs run.
> > 
> > > - In what scenarios is it necessary to specify a CPU for serial
> > > execution, or is ensuring the order sufficient?
> > 
> > I'm not sure that it's necessary.  The way I read it, at least, it seems
> > pcrypt uses cb_cpu to load balance serialization across all CPUs allowed
> > in the serial cpumask.
> 
> I recognize that the key issue isn't the capabilities it can offer, but rather
> the underlying necessity for these capabilities. If the need isn't particularly
> strong and the cost of implementing this capability is high, should we consider
> omitting it?

Well, yeah.  In this case, I'm not sure it's necessary--it looks to me
like it's not--but Steffen might have had a reason for cb_cpu that I'm
not aware of.

> As previously mentioned in another email, I'm seeking guidance on how to effectively 
> evaluate the timing of padata serialization in a network environment. 
> Could you provide some insights or pointers on this matter?

It looks like Steffen has given some hints on testing with IPsec in
another thread.