linux-kernel - Re: [PATCH v2 01/18] x86/sgx: Call cond_resched() at the end of sgx_reclaim

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Date:   Fri, 02 Dec 2022 13:37:05 -0800
From:   Kristen Carlson Accardi <kristen@...ux.intel.com>
To:     Dave Hansen <dave.hansen@...el.com>, jarkko@...nel.org,
        dave.hansen@...ux.intel.com, tj@...nel.org,
        linux-kernel@...r.kernel.org, linux-sgx@...r.kernel.org,
        cgroups@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Cc:     zhiquan1.li@...el.com, Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH v2 01/18] x86/sgx: Call cond_resched() at the end of
 sgx_reclaim_pages()

On Fri, 2022-12-02 at 13:33 -0800, Dave Hansen wrote:
> On 12/2/22 10:36, Kristen Carlson Accardi wrote:
> > From: Sean Christopherson <sean.j.christopherson@...el.com>
> > 
> > In order to avoid repetition of cond_resched() in ksgxd() and
> > sgx_alloc_epc_page(), move the invocation of post-reclaim
> > cond_resched()
> > inside sgx_reclaim_pages(). Except in the case of
> > sgx_reclaim_direct(),
> > sgx_reclaim_pages() is always called in a loop and is always
> > followed
> > by a call to cond_resched().  This will hold true for the EPC
> > cgroup
> > as well, which adds even more calls to sgx_reclaim_pages() and thus
> > cond_resched(). Calls to sgx_reclaim_direct() may be performance
> > sensitive. Allow sgx_reclaim_direct() to avoid the cond_resched()
> > call by moving the original sgx_reclaim_pages() call to
> > __sgx_reclaim_pages() and then have sgx_reclaim_pages() become a
> > wrapper around that call with a cond_resched().
> > 
> > Signed-off-by: Sean Christopherson
> > <sean.j.christopherson@...el.com>
> > Signed-off-by: Kristen Carlson Accardi <kristen@...ux.intel.com>
> > Cc: Sean Christopherson <seanjc@...gle.com>
> > ---
> >  arch/x86/kernel/cpu/sgx/main.c | 17 +++++++++++------
> >  1 file changed, 11 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/cpu/sgx/main.c
> > b/arch/x86/kernel/cpu/sgx/main.c
> > index 160c8dbee0ab..ffce6fc70a1f 100644
> > --- a/arch/x86/kernel/cpu/sgx/main.c
> > +++ b/arch/x86/kernel/cpu/sgx/main.c
> > @@ -287,7 +287,7 @@ static void sgx_reclaimer_write(struct
> > sgx_epc_page *epc_page,
> >   * problematic as it would increase the lock contention too much,
> > which would
> >   * halt forward progress.
> >   */
> > -static void sgx_reclaim_pages(void)
> > +static void __sgx_reclaim_pages(void)
> >  {
> >         struct sgx_epc_page *chunk[SGX_NR_TO_SCAN];
> >         struct sgx_backing backing[SGX_NR_TO_SCAN];
> > @@ -369,6 +369,12 @@ static void sgx_reclaim_pages(void)
> >         }
> >  }
> >  
> > +static void sgx_reclaim_pages(void)
> > +{
> > +       __sgx_reclaim_pages();
> > +       cond_resched();
> > +}
> 
> Why bother with the wrapper?  Can't we just put cond_resched() in the
> existing sgx_reclaim_pages()?

Because sgx_reclaim_direct() needs to call sgx_reclaim_pages() but not
do the cond_resched(). It was this or add a boolean or something to let
caller's opt out of the resched.