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Message-ID: <64265ef8725fe_375f7e294a@iweiny-mobl.notmuch>
Date: Thu, 30 Mar 2023 21:18:00 -0700
From: Ira Weiny <ira.weiny@...el.com>
To: Zhao Liu <zhao1.liu@...ux.intel.com>,
Jani Nikula <jani.nikula@...ux.intel.com>,
Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
Rodrigo Vivi <rodrigo.vivi@...el.com>,
Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
David Airlie <airlied@...il.com>,
Daniel Vetter <daniel@...ll.ch>,
Matthew Auld <matthew.auld@...el.com>,
Thomas Hellström
<thomas.hellstrom@...ux.intel.com>,
Nirmoy Das <nirmoy.das@...el.com>,
Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
Chris Wilson <chris@...is-wilson.co.uk>,
Christian König <christian.koenig@....com>,
<intel-gfx@...ts.freedesktop.org>,
<dri-devel@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>
CC: Ira Weiny <ira.weiny@...el.com>,
"Fabio M . De Francesco" <fmdefrancesco@...il.com>,
Zhenyu Wang <zhenyu.z.wang@...el.com>,
Zhao Liu <zhao1.liu@...el.com>
Subject: Re: [PATCH v2 9/9] drm/i915: Use kmap_local_page() in
gem/i915_gem_execbuffer.c
Zhao Liu wrote:
> From: Zhao Liu <zhao1.liu@...el.com>
>
> The use of kmap_atomic() is being deprecated in favor of
> kmap_local_page()[1], and this patch converts the calls from
> kmap_atomic() to kmap_local_page().
>
> The main difference between atomic and local mappings is that local
> mappings doesn't disable page faults or preemption (the preemption is
> disabled for !PREEMPT_RT case, otherwise it only disables migration).
>
> With kmap_local_page(), we can avoid the often unwanted side effect of
> unnecessary page faults and preemption disables.
>
> In i915_gem_execbuffer.c, eb->reloc_cache.vaddr is mapped by
> kmap_atomic() in eb_relocate_entry(), and is unmapped by
> kunmap_atomic() in reloc_cache_reset().
First off thanks for the series and sticking with this. That said this
patch kind of threw me for a loop because tracing the map/unmap calls did
not make sense to me. See below.
>
> And this mapping/unmapping occurs in two places: one is in
> eb_relocate_vma(), and another is in eb_relocate_vma_slow().
>
> The function eb_relocate_vma() or eb_relocate_vma_slow() doesn't
> need to disable pagefaults and preemption during the above mapping/
> unmapping.
>
> So it can simply use kmap_local_page() / kunmap_local() that can
> instead do the mapping / unmapping regardless of the context.
>
> Convert the calls of kmap_atomic() / kunmap_atomic() to
> kmap_local_page() / kunmap_local().
>
> [1]: https://lore.kernel.org/all/20220813220034.806698-1-ira.weiny@intel.com
>
> v2: No code change since v1. Added description of the motivation of
> using kmap_local_page() and "Suggested-by" tag of Fabio.
>
> Suggested-by: Ira Weiny <ira.weiny@...el.com>
> Suggested-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> Signed-off-by: Zhao Liu <zhao1.liu@...el.com>
> ---
> Suggested by credits:
> Ira: Referred to his task document, review comments.
> Fabio: Referred to his boiler plate commit message and his description
> about why kmap_local_page() should be preferred.
> ---
> drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c | 10 +++++-----
> 1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> index 9dce2957b4e5..805565edd148 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> @@ -1151,7 +1151,7 @@ static void reloc_cache_unmap(struct reloc_cache *cache)
>
> vaddr = unmask_page(cache->vaddr);
> if (cache->vaddr & KMAP)
> - kunmap_atomic(vaddr);
> + kunmap_local(vaddr);
In the cover letter you don't mention this unmap path. Rather you mention
only reloc_cache_reset().
After digging into this and considering these are kmap_atomic() calls I
_think_ what you have is ok. But I think I'd like to see the call paths
documented a bit more clearly. Or perhaps cleaned up a lot.
For example I see the following call possibility from a user ioctl. In
this trace I see 2 examples where something is unmapped first. I don't
understand why that is required? I would assume reloc_cache_unmap() and
reloc_kmap() are helpers called from somewhere else requiring a remapping
of the cache but I don't see it.
i915_gem_execbuffer2_ioctl()
eb_relocate_parse()
eb_relocate_parse_slow()
eb_relocate_vma_slow()
eb_relocate_entry()
reloc_cache_unmap()
kunmap_atomic() <=== HERE!
reloc_cache_remap()
kmap_atomic()
relocate_entry()
reloc_vaddr()
reloc_kmap()
kunmap_atomic() <== HERE!
kmap_atomic()
reloc_cache_reset()
kunmap_atomic()
Could these mappings be cleaned up a lot more? Perhaps by removing some
of the helper functions which AFAICT are left over from older versions of
the code?
Also as an aside I think it is really bad that eb_relocate_entry() returns
negative errors in a u64. Better to get the types right IMO.
Thanks for the series!
Ira
> else
> io_mapping_unmap_atomic((void __iomem *)vaddr);
> }
> @@ -1167,7 +1167,7 @@ static void reloc_cache_remap(struct reloc_cache *cache,
> if (cache->vaddr & KMAP) {
> struct page *page = i915_gem_object_get_page(obj, cache->page);
>
> - vaddr = kmap_atomic(page);
> + vaddr = kmap_local_page(page);
> cache->vaddr = unmask_flags(cache->vaddr) |
> (unsigned long)vaddr;
> } else {
> @@ -1197,7 +1197,7 @@ static void reloc_cache_reset(struct reloc_cache *cache, struct i915_execbuffer
> if (cache->vaddr & CLFLUSH_AFTER)
> mb();
>
> - kunmap_atomic(vaddr);
> + kunmap_local(vaddr);
> i915_gem_object_finish_access(obj);
> } else {
> struct i915_ggtt *ggtt = cache_to_ggtt(cache);
> @@ -1229,7 +1229,7 @@ static void *reloc_kmap(struct drm_i915_gem_object *obj,
> struct page *page;
>
> if (cache->vaddr) {
> - kunmap_atomic(unmask_page(cache->vaddr));
> + kunmap_local(unmask_page(cache->vaddr));
> } else {
> unsigned int flushes;
> int err;
> @@ -1251,7 +1251,7 @@ static void *reloc_kmap(struct drm_i915_gem_object *obj,
> if (!obj->mm.dirty)
> set_page_dirty(page);
>
> - vaddr = kmap_atomic(page);
> + vaddr = kmap_local_page(page);
> cache->vaddr = unmask_flags(cache->vaddr) | (unsigned long)vaddr;
> cache->page = pageno;
>
> --
> 2.34.1
>
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