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Subject: Re: [PATCH v2 26/33] Task fork and exit for rdtgroup
To: Fenghua Yu <fenghua.yu@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <h.peter.anvin@intel.com>,
        Ingo Molnar <mingo@elte.hu>, Tony Luck <tony.luck@intel.com>,
        Peter Zijlstra <peterz@infradead.org>, Tejun Heo <tj@kernel.org>,
        Borislav Petkov <bp@suse.de>, Stephane Eranian <eranian@google.com>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        David Carrillo-Cisneros <davidcc@google.com>, Shaohua Li <shli@fb.com>,
        Ravi V Shankar <ravi.v.shankar@intel.com>,
        Vikas Shivappa <vikas.shivappa@linux.intel.com>,
        Sai Prakhya <sai.praneeth.prakhya@intel.com>
References: <1473328647-33116-1-git-send-email-fenghua.yu@intel.com>
 <1473328647-33116-27-git-send-email-fenghua.yu@intel.com>
Cc: linux-kernel <linux-kernel@vger.kernel.org>, x86 <x86@kernel.org>
From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <57D88800.1090302@intel.com>
Date: Tue, 13 Sep 2016 16:13:04 -0700
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On 09/08/2016 02:57 AM, Fenghua Yu wrote:
> +void rdtgroup_fork(struct task_struct *child)
> +{
> +	struct rdtgroup *rdtgrp;
> +
> +	INIT_LIST_HEAD(&child->rg_list);
> +	if (!rdtgroup_mounted)
> +		return;
> +
> +	mutex_lock(&rdtgroup_mutex);
> +
> +	rdtgrp = current->rdtgroup;
> +	if (!rdtgrp)
> +		goto out;
> +
> +	list_add_tail(&child->rg_list, &rdtgrp->pset.tasks);
> +	child->rdtgroup = rdtgrp;
> +	atomic_inc(&rdtgrp->refcount);
> +
> +out:
> +	mutex_unlock(&rdtgroup_mutex);
> +}
...
> diff --git a/kernel/fork.c b/kernel/fork.c
> index beb3172..79bfc99 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -76,6 +76,7 @@
>  #include <linux/compiler.h>
>  #include <linux/sysctl.h>
>  #include <linux/kcov.h>
> +#include <linux/resctrl.h>
>  
>  #include <asm/pgtable.h>
>  #include <asm/pgalloc.h>
> @@ -1426,6 +1427,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
>  	p->io_context = NULL;
>  	p->audit_context = NULL;
>  	cgroup_fork(p);
> +	rdtgroup_fork(p);
>  #ifdef CONFIG_NUMA
>  	p->mempolicy = mpol_dup(p->mempolicy);
>  	if (IS_ERR(p->mempolicy)) {

Yikes, is this a new global lock and possible atomic_inc() on a shared
variable in the fork() path?  Has there been any performance or
scalability testing done on this code?

That mutex could be a disaster for fork() once the filesystem is
mounted.  Even if it goes away, if you have a large number of processes
in an rdtgroup and they are forking a lot, you're bound to see the
rdtgrp->refcount get bounced around a lot.