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From: George Guo To: Hengqi Chen Cc: Huacai Chen , WANG Xuerui , loongarch@lists.linux.dev, linux-kernel@vger.kernel.org, George Guo Subject: Re: [PATCH v3 0/2] LoongArch: Add 128-bit atomic cmpxchg support (v3) Message-ID: <20251126174016.000067f4@linux.dev> In-Reply-To: References: <20251126-2-v3-0-851b5a516801@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=GB18030 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On Wed, 26 Nov 2025 13:23:57 +0800 Hengqi Chen wrote: > On Wed, Nov 26, 2025 at 10:06 AM George Guo > wrote: > > > > This patch series adds 128-bit atomic compare-and-exchange support > > for LoongArch architecture, which fixes BPF scheduler test failures > > caused by missing 128-bit atomics support. > > > > The series consists of two patches: > > > > 1. "LoongArch: Add 128-bit atomic cmpxchg support" > > - Implements 128-bit atomic compare-and-exchange using > > LoongArch's LL.D/SC.Q instructions > > - Fixes BPF scheduler test failures (scx_central scx_qmap) where > > kmalloc_nolock_noprof returns NULL due to missing 128-bit > > atomics, leading to -ENOMEM errors during scheduler initialization > > > > This kmalloc_nolock_noprof() was introduced in v6.18-rc1 and has no > caller for now. > Why is this related to the sched_ext failure ? > Hi Hengqi, When running scx_central, function call chain as below: central_init->bpf_timer_init->__bpf_async_init->bpf_map_kmalloc_nolock->kmalloc_nolock ->kmalloc_nolock_noprof The function kmalloc_nolock_noprof returns NULL due to the following condition: if (!(s->flags & __CMPXCHG_DOUBLE) && !kmem_cache_debug(s)) /* * kmalloc_nolock() is not supported on architectures that * don't implement cmpxchg16b, but debug caches don't use * per-cpu slab and per-cpu partial slabs. They rely on * kmem_cache_node->list_lock, so kmalloc_nolock() can * attempt to allocate from debug caches by * spin_trylock_irqsave(&n->list_lock, ...) */ return NULL; The NULL return occurs because kmalloc_nolock is not supported on Loongarch, which don't implement cmpxchg16b. So I am giving the patch. Also I tried with debug caches(CONFIG_SLUB_DEBUG_ON=y), it works, but not a good idea. > > 2. "LoongArch: Enable 128-bit atomics cmpxchg support" > > - Adds select HAVE_CMPXCHG_DOUBLE and select > > HAVE_ALIGNED_STRUCT_PAGE in Kconfig to enable 128-bit atomic > > cmpxchg support > > > > The issue was identified through BPF scheduler test failures where > > scx_central and scx_qmap schedulers would fail to initialize. > > Testing was performed using the scx_qmap scheduler from > > tools/sched_ext/, confirming that the patches resolve the > > initialization failures. > > > > Signed-off-by: George Guo > > --- > > Changes in v3: > > - dbar 0 -> __WEAK_LLSC_MB > > - =ZB" (__ptr[0]) -> "r" (__ptr) > > - Link to v2: > > https://lore.kernel.org/r/20251124-2-v2-0-b38216e25fd9@linux.dev > > > > Changes in v2: > > - Use a normal ld.d for the high word instead of ll.d to avoid race > > condition > > - Insert a dbar between ll.d and ld.d to prevent reordering > > - Simply __cmpxchg128_asm("ll.d", "sc.q", ptr, o, n) to > > __cmpxchg128_asm(ptr, o, n) > > - Fix address operand constraints after testing different > > approaches: > > * ld.d with "m" > > * ll.d with "ZC", > > * sc.q with "ZB"(alternative constraints caused issues: > > - "r" caused system hang > > - "ZC" caused compiler error: > > {standard input}: Assembler messages: > > {standard input}:10037: Fatal error: Immediate overflow. > > format: u0:0 ) > > - Link to v1: > > https://lore.kernel.org/r/20251120-2-v1-0-705bdc440550@linux.dev > > > > --- > > George Guo (2): > > LoongArch: Add 128-bit atomic cmpxchg support > > LoongArch: Enable 128-bit atomics cmpxchg support > > > > arch/loongarch/Kconfig | 2 ++ > > arch/loongarch/include/asm/cmpxchg.h | 47 > > ++++++++++++++++++++++++++++++++++++ 2 files changed, 49 > > insertions(+) --- > > base-commit: d5ae5ac32615e4af729f0610fdc11ff4f4798aef > > change-id: 20251120-2-d03862b2cf6d > > > > Best regards, > > -- > > George Guo > > > >