From: "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>
To: "luto@kernel.org" <luto@kernel.org>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"peterz@infradead.org" <peterz@infradead.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
"mroos@linux.ee" <mroos@linux.ee>,
"redgecombe.lkml@gmail.com" <redgecombe.lkml@gmail.com>,
"mingo@redhat.com" <mingo@redhat.com>,
"namit@vmware.com" <namit@vmware.com>,
"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
"Hansen, Dave" <dave.hansen@intel.com>,
"bp@alien8.de" <bp@alien8.de>,
"davem@davemloft.net" <davem@davemloft.net>,
"sparclinux@vger.kernel.org" <sparclinux@vger.kernel.org>
Subject: Re: [PATCH v2 2/2] vmalloc: Remove work as from vfree path
Date: Tue, 21 May 2019 19:47:31 +0000 [thread overview]
Message-ID: <356bc82c5a34424ef1c34acfdc31f97900b9455b.camel@intel.com> (raw)
In-Reply-To: <CALCETrXfnkLKv-jJzquj+547QWiwEBSxKtM3du3UqK80FNSSGg@mail.gmail.com>
On Tue, 2019-05-21 at 10:00 -0700, Andy Lutomirski wrote:
> On Tue, May 21, 2019 at 9:51 AM Edgecombe, Rick P
> <rick.p.edgecombe@intel.com> wrote:
> > On Tue, 2019-05-21 at 09:17 -0700, Andy Lutomirski wrote:
> > > On Mon, May 20, 2019 at 4:39 PM Rick Edgecombe
> > > <rick.p.edgecombe@intel.com> wrote:
> > > > From: Rick Edgecombe <redgecombe.lkml@gmail.com>
> > > >
> > > > Calling vm_unmap_alias() in vm_remove_mappings() could
> > > > potentially
> > > > be a
> > > > lot of work to do on a free operation. Simply flushing the TLB
> > > > instead of
> > > > the whole vm_unmap_alias() operation makes the frees faster and
> > > > pushes
> > > > the heavy work to happen on allocation where it would be more
> > > > expected.
> > > > In addition to the extra work, vm_unmap_alias() takes some
> > > > locks
> > > > including
> > > > a long hold of vmap_purge_lock, which will make all other
> > > > VM_FLUSH_RESET_PERMS vfrees wait while the purge operation
> > > > happens.
> > > >
> > > > Lastly, page_address() can involve locking and lookups on some
> > > > configurations, so skip calling this by exiting out early when
> > > > !CONFIG_ARCH_HAS_SET_DIRECT_MAP.
> > >
> > > Hmm. I would have expected that the major cost of
> > > vm_unmap_aliases()
> > > would be the flush, and at least informing the code that the
> > > flush
> > > happened seems valuable. So would guess that this patch is
> > > actually
> > > a
> > > loss in throughput.
> > >
> > You are probably right about the flush taking the longest. The
> > original
> > idea of using it was exactly to improve throughput by saving a
> > flush.
> > However with vm_unmap_aliases() the flush will be over a larger
> > range
> > than before for most arch's since it will likley span from the
> > module
> > space to vmalloc. From poking around the sparc tlb flush history, I
> > guess the lazy purges used to be (still are?) a problem for them
> > because it would try to flush each page individually for some CPUs.
> > Not
> > sure about all of the other architectures, but for any
> > implementation
> > like that, using vm_unmap_alias() would turn an occasional long
> > operation into a more frequent one.
> >
> > On x86, it shouldn't be a problem to use it. We already used to
> > call
> > this function several times around a exec permission vfree.
> >
> > I guess its a tradeoff that depends on how fast large range TLB
> > flushes
> > usually are compared to small ones. I am ok dropping it, if it
> > doesn't
> > seem worth it.
>
> On x86, a full flush is probably not much slower than just flushing a
> page or two -- the main cost is in the TLB refill. I don't know
> about
> other architectures. I would drop this patch unless you have numbers
> suggesting that it's a win.
Ok. This patch also inadvertently improved some correctness in calls to
flush_tlb_kernel_range() for a rare situation. I'll work that into a
different patch.
Thanks,
Rick
next prev parent reply other threads:[~2019-05-21 19:47 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-20 23:38 [PATCH v2 0/2] Fix issues with vmalloc flush flag Rick Edgecombe
2019-05-20 23:38 ` [PATCH v2 1/2] vmalloc: Fix calculation of direct map addr range Rick Edgecombe
2019-05-20 23:38 ` [PATCH v2 2/2] vmalloc: Remove work as from vfree path Rick Edgecombe
2019-05-21 16:17 ` Andy Lutomirski
2019-05-21 16:51 ` Edgecombe, Rick P
2019-05-21 17:00 ` Andy Lutomirski
2019-05-21 19:47 ` Edgecombe, Rick P [this message]
2019-05-20 23:46 ` [PATCH v2 0/2] Fix issues with vmalloc flush flag Edgecombe, Rick P
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